Once these storage units pass a certain age, they become unreliable.
For example, if a hard disk has failed then it can be replaced, however some other component could fail next.
The best option is to replace it with a new one, as it will not make a reliable backup, even if repaired.
However, if you really do want to get it repaired, then there are several independent companies who can repair it for you, e.g. NCE: http://www.ncegroup.com/repair-and-brokerage/repair-center/data-storage-repair.html