Should I reformat a disk if Disk Utility asks me to? I have a FAT32 external disc that was suddenly unmounted as I was using it. When I reconnected it and tried to Repair it in Disk Utility, I got the following messages:

Verify and Repair volume “COMMON”
Checking file system** /dev/disk3s1
Can't open (Resource busy)
Volume repair complete.
Updating boot support partitions for the volume as required.
Error: Disk Utility can’t repair this disk. Back up as many of your files as possible, reformat the disk, and restore your backed-up files.


I then tried to do only a Verify, which succeeded:

Verifying volume “COMMON”
Checking file system** /dev/disk3s1
** Phase 1 - Preparing FAT
** Phase 2 - Checking Directories
** Phase 3 - Checking for Orphan Clusters
29576 files, 36220416 KiB free (1131888 clusters)


I then re-tried the Repair, which succeeded:

Verify and Repair volume “COMMON”
Checking file system** /dev/disk3s1
** Phase 1 - Preparing FAT
** Phase 2 - Checking Directories
** Phase 3 - Checking for Orphan Clusters
29597 files, 36219680 KiB free (1131865 clusters)
Volume repair complete.
Updating boot support partitions for the volume as required.


Does the fact that the Verify and the latter Repair succeeded mean that the filesystem is in a 100% good state? Or is the verification not exhaustive, which means that it's safer to reformat the disk?
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.