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).
Open the case, if the cable looks like this:
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then its IDE.
If it looks like this:
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Then its sata.
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