I've found two related commands in my history:
857 sudo fsck.vfat -aftvVw /dev/sdb
858 sudo fsck.vfat -avw /dev/sdb

Probably the second one will be enough.

Steps which I've preformed.

1. Connect device to some linux machine.
2. Select "Mass storage" mode on the device.
3. Open console and type "dmesg".
4. Look for follwoing info:
[ 1983.827849] USB Mass Storage support registered.
[ 1984.828702] scsi 6:0:0:0: Direct-Access Nokia Nokia N9 0316 PQ: 0 ANSI: 2
[ 1984.964233] sd 6:0:0:0: Attached scsi generic sg3 type 0
[ 1984.967442] sd 6:0:0:0: [sdb] 18463744 512-byte logical blocks: (9.45 GB/8.80 GiB)
5. Remember the device name. In my case it's /dev/sdb. In your case it can be other descriptor.
6. In console type "sudo umount /dev/sdb". Again it can be other sd in your case.
7. Run in console "sudo fsck.vfat -aftvVw /dev/sdb".

All these command fixed my device. I didn't have to format the device.

Also you can check dmesg in console on your device (in dev mode). You should get some info about locked home partition. In my case there was information that read-only flag was set. There should be even a tip to use fsck to fix issues.

If you don't have access to linux machine try running ubuntu from live-cd.
I can be wrong, but there is a chance that dummy scandisk under Windows should fix it.

Hope this helps.