Hey guys! I just had the same problem with my external hard drive, a Lacie Rikiki Go, 1Tb, that I also use for my backups.
It normally appearead on FCP X, with the projects and events stored in the external hard drive, then all of a sudden they didn't show up. I repaired the disk, via DiskUtility, twice, but yet didn't work. When all of a sudden, just moments ago as I'm typing this, it worked! I don't think it's a compatibility problem with the hard drive, just how sometimes it is connected.
For example: I use to inserd the SD card in the slot and import the files, but when it comes to delete them from the card, sometimes I can, sometimes not. It depends only on whether I open Finder first, and then navigate aroudn the card, or FCP X or Aperture first, in this case I can't move anything from the card to the Trash.
About the slowing down, it's normal. I have 8GB of RAM and yet it crashes, especially if you work on a project while it's rendering.
So just try to eject and insert the external hard drive several times, even without asking permission first.