Backing up to Airport Time Capsule

Delete the MBA’s backup entirely from TC and then start over…but I hate that solution for at least 2 reasons. You lose access to potentially important data you aren’t aware of at this time but might later and until the new backup finishes you are at risk. When it comes to my data I’m seriously risk averse. At the very least, before deleting the MBA backup on the TC, I’d back up the MBA on an external hard drive. This way you have a backup if the MBA dies in the midst of backing up. Yep - I’m belts and suspenders. If you are confident that any important file on the MBA would also be on the MBP or on the MBP’s backup then leave it at that. Back up to a hard drive. Erase the MBA backup on the TC and then back up the the MBA on the TC. Now use the external drive for whatever.

If, however, you think its possible that you deleted or edited something on the MBA that might be on the TC backup (and you have no clue where or what) then keep backing up on the external drive for 3-5 months. After that amount of time with the TC being used to back up two computers, those maybe files would have been erased by the TC’s thinning process and deletion of old backups to make room for new. Now you can safely erase the MBA backup on the TC and start using it as your backup destination. Keep the hard drive around for a few months for the same reason you didn’t immediately erase the TC.

If the error message (There's not enough space) is correct, as odd as it sounds, it is fully possible. The issue is this; TimeMachine backs up files that have changed or that are new since the last time the computer was connected to the drive. If it has been a long time since your MBAir was last connected, or, if you've made a big change to the MBA (like upgraded a program or the operating system) that would require a lot of space. Meanwhile, your MBPro might not have any major changes (or its major changes were backed up first taking up all the space).

TimeMachine should delete old backups to create enough space but there are two limitations. First, you have 2 computers backed up on the drive and I think TM will only delete the old backups of the computer it is currently backing up. I don't think it will touch old backups of the MBP to make space for the MBA. Second, I know it will only delete files that are duplicated. TimeMachine, by design, must leave everything that is needed to restore the computer as it was the last time it was backed up successfully. Sometimes TM just cannot delete enough space to fit the new backup.