I was having the same problem with an Ubuntu 12.04 and an IceBox USB device.
After googling for a while I suspected it could be a kind of timeout for the usb device so I run this command:
After a whole weekend without any problem I add this line to my crontab:
And that's it! It is not very elegant but works fine for me.
*/5 * * * * touch /USB_disk/.prueba.tmp && rm /USB_disk/.prueba.tmp