Bankrupt? whoops backed up would be used in a past tense sentence ie "Fortunately for Joe he backed up his work before the power strike." ROOFIS
Ahh, thanks! I think that'll do. Backed up. But it comes strange that some dictionaries already include backup as a verb. So it's a little bit strange that you use backed up for the past tense of a verb backup.
As a verb you'll always find this term in the present tense. You asked for the past tense so I gave an example. ROOFIS