We have 2x USB drives (WD 1TB - same brand, no external PS, 1x USB connection per drive) which are connected to an SBS 2k11 VM via USB passthrough to a Dell R420 new server. Since we thought it's a physical server issue, we've upgraded from Dell R210 to Dell R420 - same issue.
The disks seem to be connected OK, until they're disconnected automatically from the server, at different times, usually during of Windows backup, but not necessarily.
I'm attaching 2 screenshots with the events.
ESXi version - 5.1 U2, Free Ed.
Your assistance will be more than welcome!