Checkmate alerted me that my Time Machine external drive had a damaged partition map. I consulted Checkmate Help and it said that Disk Utility could repair this, which it did. Since the TM drive has a lot of data (1.86 TB filled out of 2 TB), DU took a long time (maybe about 45 minutes), mostly spent on analyzing things that weren't a problem.

Is there any way to selectively repair the partition map without having to go through all the other tests?

EDIT: I found a possible answer here but I don't know if it's accurate (scroll down to "1 Answer").