"[Library] is already in use."

Tried to open a library on an external drive, fcp crashed.
 Ran disc utility, restarted fcp, tried to open the library, it said "The operation couldn’t be completed. Permission denied, “[library name]” is already in use by “[my name]” on “macintosh-d49a2006fa98.local”. I tried both trashing and deleting Prefs. I was then able to open other libraries on the ext. drive but not the one I want, FCP keeps crashing when I do or I get the "already in use" dialogue.

Had this same problem. In my case, I use an external hard drive to back up all my FCP files since they build up quickly and become gargantuan. I didn't realize my hard drive was filling up with very little space left. As soon as I moved some files off the drive to allow at least 200MB left on the drive, everything worked just fine again.