It sounds like when you inserted it in that "other device that didn't work" that maybe it got hosed by THAT device. If the Droid X won't allow you to Mount it, you can't reformat it on the phone. Do you have access to another SD Card reader? like in your PC or Laptop? Might be time to stick it in another reader that can at least reformat it .. of course any data you had on the card is probably gone.
Disconnect your phone from USB if you wish to mount the drive on your phone. Also, I've seen software conflicts that cause the SD to not mount on the phone but I can't always replicate the problem. If the SD says "Unavailable" and you are not plugged into USB goto the Apps menu, touch the Menu button, Manage Apps, select the Running Apps tab and close Backup Assistant + Media. When it is closed see if your card shows up. If not try closing any other running app that have drive mount permissions. I did this once, drive showed up, rebooted the phone and it hasn't done it again. I'll post again if I come up with anything new.