I wouldn't worry about it. You probably would get an error message if you tried to unmount the boot volume ("C" drive), but I wouldn't test it just the same.
New build, Asus MB, Pentium D cpu, XP pro, install fresh from CD, then MB disk with chipset and other drivers, everything in, on, working, all updates ok, clean everyway I can think of, plugged in an external USB hd for some archives, and when disconnecting i find that the "C" drive shows as disconnect available!! Never heard or seen such a thing. The install to the Sata drive was smooth, no problems at all. Is this a problem, how could it be corrected ? TIA