I know this comes a couple of years after the original post, but in case anyone else should come along with the same problem, it is possible that if your external drive is not showing up in the Events area of FCPX, but it is mounted on the Mac, then perhaps the permissions are not set correctly. User sumitrana, in another thread, pointed out this permissions issue.
Go to the Finder and select the drive in question. Press Command-I to Get Info. Take a look at the permissions section at the bottom. If your own username is not there with "read and write" ability, then add yourself to the list of permitted users, or at least set yourself to both read and write if you are already there. Don't mess with any other "users" (such as "wheel" or "system" or "everyone").
Then, quit FCPX and eject and remount the drive (I used Disk Utility to do the latter and did a housekeeping Verify on the drive before I remounted just because I could at that point). Fire up FCPX again and all should be well.
Good luck.