When this first happened to one of my external backup drives, I was at a loss. The drive would not show up in Finder, but, I could see it in the “Disk Utility”. Attempting to repair the disk from Disk Utility, resulted in the message above.
Naturally, I Googled around and found that this was a very common issue and quite a few people had posted similar issues on Apple and other forums. Mostly people put it down to WD hard drives and some had even written to WD about the same.
After some experimenting, I found the solution to be surprisingly simple and pointed to yet another bug in Mavericks.
I tried this on different external drives of different capacities by simply pulling out the USB without ejecting the drive. Sure enough, I got the same issue and message after running disk utility on almost all occasions.
The solution, for me, was quite simple. Let the “Disk Utility” try and repair the drive and come up with the message shown above. Then remove the drive and plug it back in and it will work. Just run repair on it again to make sure that all is as expected.
Keep in mind that it might take around 2-10 minutes for the drive to show up in “Disk Utility” for the first time and it will not auto-mount and show up in Finder. Keep the “Disk Utility” running. After that, it’s simple. Run repair. Ignore the error. Re-plug the drive and run repair again.
I have tried this on 3 WD and 2 Seagate drives with HFS and ExFAT, all work. Hopefully it would work for some others who have similar issues.
Nitin Chandra's Blog 