Oh, or maybe ESPs are only used in EUFI systems (my solus being a BIOS), so it wouldn't have an ESP that I could copy the windows ESP contents to...

https://getsol.us/articles/installation/disks/en/
the big quote is from this link. This pretty much is how Solus says your situation must look like before attempting any install next to windows.
I should shut my yap now since I have never attempted this, but the ESP delete looks scary.

"Solus provides multiple ways you can install onto your system.

1)Solus can take up the entire drive, using traditonal root partitions, or with LVM-managed root partitions.
2) You can install Solus next to your existing operating system, such as Windows. If you have multiple operating systems installed, we will choose the largest option.
3)You can manually configure your system. This is only recommend for advanced users which may desire to have a dedicated /home partition. Note that if you manually configure your system, you will need to restart the Installer for those changes to apply appropriately. Note: Legacy (BIOS) must use MBR partition table while Unified EFI (UEFI) must use GPT partition table.
Alongside these options, we also provide the ability to use full-disk encryption. For the obvious reason of it being full-disk, this assumes you are installing Solus onto the entire drive.

UEFI
If you are using a system with UEFI, you may need to create a EFI System Partition, also referred to as an ESP. This is not necessary if you are enabling Solus to install onto the entire disk.

To create an EFI System Partition, open up GParted and create a FAT32 partition that is 512MB in size. Next, right-click on the partition and click Manage Flags. On the Manage Flags section, enable the boot and esp flags.

Notes:

Your system must be booted using Unified EFI (UEFI) mode, as opposed to a “legacy (BIOS) mode”.
Secure Boot needs to be disabled.
Your disk is required to be GPT formatted.
If you cannot see your SSD drive, set the SATA configuration to AHCI."

This is the condition you are supposed to begin installing in. I hope one of the forum users who has done this successfully will chime in.
edit: the bullet points did not survive cut and paste

yeah, that's requiring a EUFI set up, which mine isn't.. So that would bring me back to the question of can I upgrade from BIOS to EUFI, to start at that point (or, ofc, reinstalling..)

https://unix.stackexchange.com/questions/252936/grub2-boot-to-a-second-another-hard-disk

From this, trying user os-prober to update Grub. I do have a place to edit grub, so I'm just gonna assume that's what I'm using, as I'm not on EUFI... since Solus only seems to offer much advice if you're using a EUFI, so there's not much I can do to get around that it seems, shy of a clean install.. Hoping to find a way that doesn't require that, but eh.. we'll see.

It said it recognized windows 10 but didn't give me the option to boot into it

Oh, it says to put in this afterwards: grub2-mkconfig -o /boot/grub{2}/grub.cfg

However, Konsole says "grub2-mkconfig: command not found"

Over here boutta break something I think buuuut gonna just remove 2 from that.... so it's

grub2-mkconfig -o /boot/grub{2}/grub.cfg

Hm. Well, command found, but no file found....


Try just

grub-mkconfig.

oh, that did something. Lots of words... Time to cross my fingers reboot and see what happens, I suppose!

Still no ability to boot into windows, although I forgot to mention I am not having it crash after plugging in the Win10 HDD anymore, it just directly boots into Solus so that's nice.

Few reboots messing around with trying to boot into second HDD via f12, for some reason it just kept not letting me do that... Finally went ahed and ran os-prober than grub-update (...or was it update-grub? Uhh..)

Anyways I can see windows 10 now!!

Unfortunately, it's still having the issue of the actual windows 10 partition is messed up, but that's possibly a seperate issue.

okay, to summarize for anyone else trying to replicate what I've done in the future (also, hi future me! You messed something up, and forgot how you fixed this, huh? well don't worry..)

I would say try, in the console

sudo os-prober

then

sudo grub-mkconfig

followed by yet another

sudo os-prober

(no, I don't think this should be necessary, yes it did work after I did it this way, feel free to skip if you want, I guess?)

then lastly

sudo update-grub

I have had trouble dual-booting on separate drives in the past because, sooner or later, the boot managers get tangled.

I've found a simple (if tedious) way to dual-drive, dual-boot successfully. Here is what I do:

(1) I back up all the data and anything else I want to keep (e.g. Windows installation executable files) I want to keep onto an external USB or external drive.
(2) I create a USB installer for Windows using the Media Creation Tool (Windows 10 or Windows 11, as the case may be).
(3) I create a USB installer for Solus (in my case using Rufus on Windows).
(4) I boot into BIOS (F2 on Dell computers) and remove all boot managers (Windows Boot Manager, Windows Boot Manager, Grub, whatever). I also make sure that the computer is set up correctly for a Solus install (UEFI, Secure Boot disabled, AHCI rather than RAID, and so on).
(5) I remove** all but one drive from the computer.
(6) Using the Media Creation Tool USB, I do a "destructive" clean install of Windows on the drive in the computer. By "destructive", I mean that I use the "Custom" install option for installation (part of the Windows Media Creation Tool USB installation process), deleting all partitions on the drive as part of the installation process. As a result, Windows installs on an absolutely clean, blank drive with no prior partitions, including boot partitions.
(7) I get Windows set up with drivers and whatnot so that the computer boots properly into Windows with no hardware issues.
(8) I boot into BIOS and check the boot manager to make sure that the only boot manager is the Windows Boot Manager.
(9) I remove the Windows drive from the computer. I put the other (soon-to-be Solus) drive into the computer. At this point, I again have only one drive in the computer.
(10) I install Solus on the drive in the computer, again using the clean "destructive" installation technique (in Solus, the option is to "Completely erase the drive ..." rather than "Install alongside ...") so that I am installing Solus on a clean hard drive.
(11) I get Solus set up with drivers and whatnot so that the computer boots into Solus with no hardware issues.
(12) I boot into BIOS and check the boot managers (should be Windows Boot Manager and Linux Boot Manager) are in place, and I set whatever boot manager I want as default (in my case, Linux Boot Manager for Solus) to be the default bootloader.
(13) I put the Windows drive back into the computer. At this point, I have two drives (one Windows, one Solus), each with an independent, completely separate boot partition, not seeing each other at all. The BIOS will boot into the default OS (in my case Solus) without intervention (F12 on Dell computers) but will boot into the non-default OS (in my case Windows) using F12 or whatever other key accesses the BIOS boot menu. Because the bootloaders (Windows and Solus) are on separate drives and do not interact, the bootloaders don't corrupt each other or get tangled. In essence what I am doing is building two computers (one Windows and on Solus) intersecting only at the BIOS boot menu.
(14) I boot into Solus and set the OS up the way I want to set it up (apps, data, appearance, settings and so on).
(15) I boot into Windows and set the OS up the way I want to set it up (apps, data, appearance, settings and so on).
(16) I start using the computer.

FOUR NOTES;
(A) I have never tried to set up a computer this way with any distro other than Solus. So my comments above are limited to a dual-boot, dual-drive Windows/Solus installation.
(B) I install Windows first and Solus second. I have no reason for this, other than habit. I can't see any reason why it would not work the other way around (Solus first, then Windows), but I have no experience with that method.
(C) If you set up Solus (rather than Windows) as your default boot OS (as I do), then you will need to be careful going forward handling Windows updates and attendant restarts, being careful to always use F12 to restart into Windows after an update. If you don't, the BIOS will default to Solus, opening it, and the Windows restart process does not proceed normally. My own practice is to use the "Update and Shut Down" option instead of "Restart" in that situation (see NOTE (D) below) because it reduces risk of error.
(D) I disable "Fast Start" (or whatever it is called these days) in Windows, so that power off doesn't try to save the system state. I don't have a technical reason why this is necessary in a dual-drive dual-boot environment with separate bootloaders (as described above) but I prefer to have Windows completely shut down when I shut it down because that is what I want it do do when I "Update and Shut Down".

===========
** In the case of an M2 SATA or NVMe drive or a SSD or HHD that is installed into a slot (as in the case of a laptop or micro desktop), the drive will need to be physically removed. In the case of a SATA SSD or HHD in a larger desktop, the drive can be "removed" by unplugging the cable to the motherboard.

    So I'm a little late to this but I do things similar to tomscharbach. I had Solus installed first on a m.2 and installed windows on a separate SATA SSD. The BIOS for my Lenovo is not great, so I manage boot entries with efibootmgr in Solus, and it's also set to be my primary boot source. Both are EFI, I have fast boot enabled, and haven't had any issues.

    I think the key is having the non-target drive mechanically removed from the mechine before installing the OS. I've done a few machines this way with Solus/Windows and other distros and Windows and haven't run into an issue yet. The only issue I have is sometimes Windows wants to take over as the primary boot, but usually pressing F12 and booting into Solus once fixes that, I don't have to play with boot order in the BIOS.

      I use to do that years ago got in habit when setting up drives to take the ones out not using just so I didnt mess one
      up by being stupid and lose data..lol. Nowdays I have to many computers so I just keep things seperate.If you are only one using it usually not an issue. I even tried those slide in carries on the desktop and just popped in the drive and os I wanted to use but never did find one that took the abuse but that was years years ago maybe those slide carriers have changed if you can still buy them plus they never looked to good.. (Just ramblin..lol)

      Heh... yeah, if removing a drive wasn't a 30 minute ordeal I'd do that, but my case sucks, the placement of my PC sucks, and that's a massive hassle when I'm just trying to play one of the few games that doesn't work on Solus..

      Removing my drives during install would've been smart though. Everything seemed to have worked fine, until this morning when I realized that Windows altered my Data HDD in some way and now Linux can't write to it... so that's gonna be a whole thing to deal with, doesn't look like Windows touched my Solus SSD though so hey, it could've been worse... er, in fact gonna have to make a new post on that (if I can't fix myself first).. it's not just my data drive doing that it seems 😅

        Oh, silly me, I just forgot to disable fast boot in windows, all good! 😃

        CT42 ... a massive hassle when I'm just trying to play one of the few games that doesn't work on Solus

        LOL, but I'm laughing with you. I keep a circa-2017 Windows 10 Dell Inspiron 11-3180 (AMD A-9 with R5 graphics) laptop alive for the sole purpose of playing Red Alert 2 - The Aftermath (circa 1996). Try as I might, I've never been able to get RA2 running on newer Intel CPU/GPU computers, but it runs fine on older AMD computers. I have no idea what I am going to do when that computer dies or Windows 10 EOL comes, but then again I'm 75 so it might not be an issue.

          CT42 I'm just trying to play one of the few games that doesn't work on Solus..

          Have you looked into Stadia? Don't know if you have a Google adversion, but it's what I use to play Destiny on my laptop w/Solus. Works great. Of course, if you've already bought and paid for the game or dlc on steam or somewhere else, it's a non-starter probably.

              Brucehankins Suuuuper averse to Stadia, less because google per se and more because I'm not gonna pay money for something that may not exist in a few years (which, ofc, is because Google..), but that's irrelevant, the few things I can't get working on Solus aren't gonna be on Stadia lol XD Also, Geforce now > Stadia every day ;D Play what I already own, thanks!

              Most of the things I'm struggling with on Solus are small indie games/projects (like Deios by Dungeonfog, a map making software is a big one which finally convinced me to set up the Dual boot.. most of the RPG Maker games either worked well enough in a VM or I didn't care enough for the effort, but Deios was different), but I was able to get Win10 dual boot up and runnning anyhow ^.^

                tomscharbach (9) I remove the Windows drive from the computer. I put the other (soon-to-be Solus) drive into the computer. At this point, I again have only one drive in the computer.

                I understand everything in this great tutorial except this. Are you removing one drive by removing it from a bay and unplugging it from the board and putting the next drive in the same bay and same plugin on the board? Or different? Or would this (labeled board plugin slots) theoretically matter at all?
                Reason I ask: this tutorial makes me confident, and I've been running Solus alone for too many years, ergo companion OS has occurred to me lately

                    brent I understand everything in this great tutorial except this. Are you removing one drive by removing it from a bay and unplugging it from the board and putting the next drive in the same bay and same plugin on the board? Or different? Or would this (labeled board plugin slots) theoretically matter at all?

                    The computer I am using to dual boot is a Dell Optiplex 7000-series Micro. Micro-size computers are similar to laptops in configuration (limited drives, low-power CPU/GPU, and so on).

                    The Optiplex Micro has three drive slots -- two M2 drive slots (one for a 2280 "long" drive and the other for a 2230 "short" drive) and a 2.5" SATA slot with a snap-in plastic drive holder. So it is very simple to work with. I use the "long" M2 drive slot for an NVMe drive, and the 2.5" SATA slot for an SSD. I don't use the other M2 slot, so I effectively have two drives to work with. That makes the install process more or less idiot-proof, which is a good thing in my case.

                    I installed Windows on the M2 NVMe drive (back in the day when Linux support for NVMe was problematic) and Solus on the 2.5" SATA SSD drive. I popped out the 2.5" SATA drive when I installed Windows on the M2 NVMe drive, then popped it out. I put in the 2.5" SSD when I installed Solus on it, then, after Solus was installed on that drive, popped the M2 NVMe drive back into the M2 slot.

                    On a more typical desktop computer (e.g. a computer with an M2 slot or two and several SATA 2.5" SSD/HDD slots) the drive situation is more complicated, obviously.

                    I would follow the practice of setting up each of the OS drives in the bays/slots where the drive will reside at the end of the process -- that is, install Windows on the Windows-target drive in the bay/slot where it will reside permanently, and install Solus on the Solus-target drive in the bay/slot where it will reside permanently.

                    I would not set up both drives using the same bay/slot. I don't think that it would do any harm to use the same bay/slot to set up both OS's (modern motherboards seem to adjust to moving drives around**), but why do it? On a mid-size or full-size desktop computer, "removing" a 2.5" SATA SSD/HHD is as simple as unplugging the drive from the motherboard slot.

                    The three important principles are:
                    (1) Set up both installer USB drives (Windows and Solus) before doing anything else, and test them to make sure that they will boot before going further;
                    (2) Clean up the BIOS boot loader entries before installing (I don't know why, but BIOS seems to collect boot load entries, and when I install too many different OS's on my test computer, boot loader entries multiply like rabbits); and
                    (3) Unplug or remove all non-target drives when installing the OS on the target drive.

                    I'm going to mention Brucehankins so he can chime in if he has any thoughts on this issue. He has a lot more experience that I do, I suspect, and might have a better answer than I do.

                    ===========

                    ** I remember (correctly or not -- at my age you never know) that in the 1980's and even into the early 1990's some motherboards required the boot drive had to be in a specific slot (as I recall, the CD went into slot 0, the boot drive into slot 1, and additional drives went into slots 2 and up) but I don't think that is the case with modern motherboards.