Quick fix for an annoying problem I ran across where the removable storage no longer shows the attached usb drives after a reboot under XenServer 5.6. Pop open a console window on your XenServer host:
modprobe -r usb_storage … this removes the usb_storage kernel driver
modprobe usb_storage … this reinstalls the usb_storage kernel drives
That should get you your drives back and if you don’t see them then just do a rescan.
xe sr-list | grep -i removable -B 1 … use this to find the UUID of your removable storage SR
xe sr-scan uuid=<uuid of removable storage> … your usb drives should be showing up now ready to be attached to your VM