https://discourse.gnome.org/t/disk-size-unknown/21968/6
guy named jake says try gnome-control-center --verbose system
so why doesn't the gnome front end see disk capacity? not updating my gnome laptop til tomorrow so cannot duplicate this yet. be interested if someone could corroborate.
couldn't find an issue here at forum. didn't check devtracker
according to ddg, search: "disk capacity unknown gnome settings linux" it's a very well known Gnome issue.
solved thru updates, funny handshakes, a ton of reasons and not fixed for a ton of reasons...surfing a little leads me to believe it's just a plain old fashioned bug. so known it's too much reading. but scanning about 12 threads don't really see any "ta-da" fixes at all.
report it here: https://github.com/getsolus/packages/issues is my 2 cents, and good question