After a reboot or shutdown the VM loose the connection with the USB HDD

First: I am sorry for my English, I am not a native speaker.
Second: We never had this issue with XenServer 6.5, with or without the SP1 and subsequents updates.
Third: We did a clean install of Xenserver 7.

That server holds just that VM, that is used to backup our network files, saving the files locally and in the USB HDD, that we carry home every week.
When the server and the VM goes up again, after applying an update that demands a reboot, or after a shutdown caused by a power loss in the weekend or dawn, the USB disk disappears from inside the VM and from the "Removable Storage".
Doing a rescan do not finds the disk at all.
One or two times, however, the disk remained listed in the "Removable Storage" after the reboot, but not inside the VM (don't know why...) and when we tried to attach it to the VM it failed with the message "The VDI is not available".
The only workaround to this problem is to change the USB port after every reboot, what is really boring and annoying.