If it's SATA it could be down to one of the settings.
From memory SATA has "removable" drives as part of the spec (ESATA is basically just a different connector), so if the controller supports hot swap it'll show as a removable drive, even if it's an internal one.
I think there is a way to hide that option for specified drives in windows though

(I've got all 4 of my SATA drives showing as removable).