Recently I tried to plug in a 2 TB usb hd to my NAS, and couldn't get it working. Even manually mounting it failed with "invalid argument" errorcode even though the drive was listed correctly by fdisk.
Since filesystem on the disk was NTFS, I thought installing the ntfs-3g driver would help. After installing thru ipkg, I tried to mount the drive with ntfs-3g. It said due to a bad windows unmount, filesystem was damaged. It automatically fixed it and everything worked ok since then.
So when you get the invalid argument error while manually mounting a drive, check the filesystem integrity, it might work.