Should I reformat a disk if Disk Utility asks me to?
Since the error that showed up the first time was just Can't open (Resource busy), I wouldn't worry about it. It didn't actually do anything the first time. I'm guessing you had a file open or something and it couldn't unmount the volume to repair. I think the reformat message shows up if a repair fails, for any reason, even if it has nothing to do with the filesystem.