If you can't delete the filesystem is corrupted somehow. Sounds like you need to restore the content to the 622 then wipe and reformat the external drive. or just consider it a "permanent" storage device as you said.
I agree.
Pick up another 500 gigger. You'll need it eventually anyway. Move the content and reformat the first.
These problems may be caused by touchy USB communication. Hopefully E* will look into this and add some checks to the storage system.