For some this problem might be the result of a corrupted MBR + partition table.
I had my USB key formatted with VFAT, which doesn't seem to be a very good choice. After using it a few times only, automount stopped working and fdisk wasn't able to access the partition anymore.
Unless Windows compatibility is required, I suggest formatting the drive with EXT2/EXT3. At least, I never had a problem with that.
For some this problem might be the result of a corrupted MBR + partition table.
I had my USB key formatted with VFAT, which doesn't seem to be a very good choice. After using it a few times only, automount stopped working and fdisk wasn't able to access the partition anymore.
Unless Windows compatibility is required, I suggest formatting the drive with EXT2/EXT3. At least, I never had a problem with that.
I solved the problem with:
dd if=/dev/zero of=/dev/ {your_USB_ device} bs=512 count=1
After that I was able to access the drive again and rebuilt the partition table.