Some times when USB drives have been plugged in for a long time they seems to disappear. Normally that’s not a problem you just unplug them and plug them back in. When the USB drive is in a Data center that’s a long way to drive just to unplug a drive and plug it back in.
For Xenserver try
modprobe -r usb_storage
Unplugs all USB storage from the server
Plugs it back in. Normally a re scan on the Removable storage put everything back right.