the Seagate HDD comes pre-partioned? That doesn't seen normal. Smells like a refurb,
Either way you will know if your GPT formatting works when you install an OS on it successfully.
If this was me I would nuke the whole HDD, partitions, data and all, and re-do it pure GPT. It's hard to tell if there's some relationship between the two partitions.
A weird problem to be sure.
