Not same problem, but, had one of my TM drives backing up real slow and always 1.2GB. Thought maybe drive was going bad, speed tests IO were slow, but no issues found via Disk Utility. After erasing and reformatting, all is well with it since. Was not a big deal, for me, losing the old backups as I have a second TM drive and probably forgot what might be on there from ages ago (read: probably not important at this time as the really important stuff was backed up via other ways/places).