I was using Rename, Move, Copy in MC to move 600 files from a USB drive to a SATA drive, and half way through the USB drive disappeared! It seems like MC tagged all 600 files but only half actually got moved. The USB drive is now working and I can access all those files in windows explorer, but how do I get MC to find them again without using Import (and probably loosing all the tagging)?
So, step one is to either disable auto-import in the background (if you normally leave it on), or turn off the "fix broken links" auto-import option temporarily. This is to prevent MC from removing your broken links before you can fix them yourself.
Then go to a view that shows the files you previously tried to move, can you see which links are "broken"? If so select one or two of the broken links, and then open the rename, move, copy tool. In the upper left corner, there's a drop down, and you should choose the fourth option (the one that says something like "update database to point to new location"). This tool can be used to change what file location the library database entries point to without actually moving any files. Because the database entries were changed, but the underlying files weren't moved to the new location, you need to use the "update" tool to do the reverse of whatever previous move you did (i.e. if you were previously trying to move from C:\ to D:\, instead go from D:\ to C:\).
Once you do that the broken library entries should now be pointing back at the original file location and the red broken link icon should disappear. If the files play when you click on them, that's a good sign. If that worked correctly, you should then be able to do the original rename, move, copy operation on those files and they should get moved properly.
Once you've gotten it working with a few files, then you can go back and do all the broken links at once.