On the 211 press Menu - 9 - 2 to disconnect the EHD, don't just unplug it, it may corrupt the OS on the drive and make it reformat when you try to reconnect.
When they first introduced the EHD on 211, there was no support and I learned the hard way that just unplugging the drive or just disconnecting the USB can really screw with the OS, because the 211 is constantly buffering data to the drive, it needs to be disconnected correctly to ensure success.
Initially there was no option aas mentioned above, You hav to shut doen the 211k (unplug it) before you could safely remove the EHD.
The first one I disconnected I had to reformat it on my PC then have the 211 reformat it before it would work on the 211 again - really messed up the OS on the drive.