I have an older USB2.0 300GB external drive (Maxtor) that works fine (read/write) when connected via the PC's USB port. When plugged in via the back of the Linkstation it comes up with a write protection error every time you try to run a backup - in fact, even accessing it through windows rather than the LS firmware you can read files but not write to it!
What's going on!?
You have to reformat the USB drive to XFS (linux). This can be done if the LS is picking up the USb drive in the user interface. You probably have the drive formatted to NTFS currently so I would suggest reformatting it to FAT32 then plugging it into the LS. Remember that reformatting your USB drive will erase the data that is on it.