First I would make sure that your iMovie Projects folder is at the top level of the drive. Also the iMovie Events folder must be at the top level of the drive. If you have them inside any other folder, iMovie will not see them.
(The exception to this is on your startup drive. There, iMovie expects to find these folders in the Movies folder.)
If that does not work, I would recheck the formatting of the drive. Instructions for formatting are here.