decrypting backup disc not allowing access

My external backup disc has been decrypting for some time, even tho' I hadn't requested it to. that seemed ok but now when I plug in I get a box asking for password required to turn off encryption". I put in the password and a box says "error preparing the Time Machine backup disk" "This core storage logical volume already decrypting" I ok it, try to use the disc again and the loop starts again asking for password.

So I can't do a backup. will it sort itself out if I leave it plugged in to decrypt even tho it shows now indication of doing so?