Well, if you're using the SATA power connector, and your motherboard supports "hot swap" features for SATA (I believe it does) it would certainly explain it
I believe this to be a bug with Nvidia drivers. Under certain circumstances, hot swapping a SATA drive wouldn't be that bad. However, you should NOT be able to "safely" remove a drive containing your active/boot partition. Nvidia has some work to do on their drivers I see (NCQ issues as well).