Waaay back when I got my first 211 I tried unplugging the hard drive while it was still turned on and it corrupted the OS on the hard drive. I had to reconnect it to a PC, wipe the partitions, rebuild the Windows partitions, then it would accept the drive to reformat to its liking. So, I never just unplugged the drive after that. Always turned the 211 off before disconnecting. Dish may have a more sophisticated OS on there now that can tolerate live disconnects.
IIRC, the reason it must reboot after disconnecting the hard drive is because the OS that knows how to record is actually on the hard drive and without the hard drive it reverts to an on-chip OS that is much simpler. This is also part of the reason for the incompatibility between the EHD's from the Hopper/722/622 and the 211/411.