From reading similar threads, and from my own experiences, the prevailing wisdom seems to be that:
* the drive must be formatted as Mac OS Extended (Journal), not FAT32, NTFS or other
* the drive must not be currently used as a time machine disk
* (theory) the drive must not have previously been used a time machine disk
From my own experiences, I had 3 externals that worked w/ iMovie 11 but not w/ FCP X. With some experimentation, I was able to confirm the above items.
- one drive was a TM disk, and just wasn't ever going to work. Annoying, but at least an identified issue (or bug)
- one drive was once a TM disk but I'd stopped using it as such. I'd manually deleted the backups on it, but suspect that TM likely leaves invisble files around to identify meta on file lists, exclusions, etc. Showing invisibles in finder listed a plethora of files that I wasn't willing to research. Went into disk utility, formatted as Extended Journaled, and the disk started to appear in project and event windows in FCP X.
- one drive was formatted as NTFS. I use Tuxera to read/write NTFS and this seemingly caused no problems in iMovie. With numerous threads discussing FAT32/NTFS incompatability, I opted to format this disk and then restore my old iMovie events and projects. Again, this resulted in the drive being visible in FCP X.
One note, as mentioned above, in the case of the disk wehere I restored old iMovie events/projects, the disk didn't immediately show in the events window (but did show projects immediately). The solution was to insure that the events window was grouping events by disk. The default layout hid away my external disk until I did this--and once grouped properly, I was able to then import my iMovie events into FCP X successfully.
Good luck...the support for exernal drives (and likely NAS) seems really poorly documented. I lost at least two days to this.