Past tense of "backup"

Solution 1:

Backed up is the correct form. I never saw anyone spell it backuped or backup-ed until today. Also see the n-gram chart.

Solution 2:

Backup is a noun when it is written adjoined. Its verb form is back up (note the space between back and up). Don't mix these two.

That said, the past tense of back up is backed up.

Solution 3:

The noun a backup is derived from the verb to back up. Therefore, the past form should be backed up.

If it was the other way round -- if to backup was a verb derived from the noun a backup -- then backuped would be correct. That's probably how you became confused. Somehow, in your mental dictionary, backup / back up became mis-categorised as a verb derived from a noun.

Solution 4:

I would say backed-up, although from personal experience, mistaken, let-down and screwed are more honest.