Step 1, reenter where you want the media, render files, and FC backups kept using the first dialog box that opened.
Step 2, consolidate media to that same folder (drive) - highlight the library, then in the inspector info tab, you'll see the consolidate button. If you are saving everything to the library, that's the default option.
Step 2, reimport all your original media from wherever you are saving the media.
If it is a permissions issue, select the drive that is no long granting permission, then get info (command-I) , unlike the lock at the bottom right, and see if you have read-write permission. Including all enclosed folders If you do, then repair permissions.
Most of the time if I get this error, a re-import will solve the problem. And I just reimport everything. FC will only update missing stuff and it will do it much faster than the time it took to do it the first time. I find it less hit or miss than trying to use " re-link files,"
Are you sharing the drive/media with another editor?
Mac Pro, macbook pro, Imacs (i7); Canon 5D Mark III/70D, Panasonic AG-HPX170/AG-HPX250P, Canon HV40, Sony Z7U/VX2000/PD170; FCP 6 certified; FCP X write professionally for a variety of media; teach video production in L.A.