Maybe somebody still have a problem... Yesterday I spend half a day to solve this situation on my F320BT:
I think that for U310BT all the same.
So you need:
1) Connect navi to PC by the usb cable
2) Enter Navigation menu -> PC Connect. Navi will connected to PC and you can see sd drive in PC
3) Format card in this new drive. (I use 4GB SDHC on FAT32). After formatting you find a new folder on sd card with device.dat file. Do not delete it ever.
4) run NavGate feeds and do what you want with your sd card
5) press "Restart" on Navi to exit PC-connection mode.
