Failed backup/restore to/from USB disk

Since I've upgraded my Fortigate (FG40C, FG60D, FG110C...) with FortiOS 5.2.2, I'm unable to backup my configuration to USB or restore a config from my USB disk.

The CLI command used is "execute backup config usb myfilename.conf" or "execute restore config usb myfilename.conf".

I always receive the following error message : "No usb disk plugged-in, please wait... command fail. return code -160".

Executing the following CLi command "execute usb-disk list", the Fortigate is able to display the USB files list.

The backup/restore works fine with the same USB disk (FAT formatted), on the same Fortigate appliance with an older firmware version (4.0 MR3 Patch 10 for example).

Any idea ?

Thanks for your answers...