A vexing problem no doubt. I take it that you have rebooted? It is unlikely that the cord has gone bad, unless you had some little furry creatures chewing on it. They usually get disinterested in it once they hit the coax sheath of a firewire cable, preferring bird food or even belly button lint by a wide margin. Assuming that it is connected back up to the MMini and is powered, and the MMini boots up, do you notice any blinking of the drive light during that process?
Ooh. 4 pages to catch up on. I'll answer this first.
It's been a while. I've rebooted several times for a variety of reasons (including the whole thing slowing down and wanting more memory to play with.) It isn't mice. My MMini and external drives are on shelves about 4 feet off the ground.
One day I just noticed it wasn't showing up. On the side of my desktop there is the hard drive, my Time Machine and my Video Back Up. (The last 2 are on a partitioned 1TB disk.) This is a WD 500GB disk. The white light runs up and down as if it is working. When I unplugged it last night, my computer yelled at me for not dismounting it first but there was nothing to dismount -- how do you tell it to disconnect something that it thinks isn't there?
If it plugged into a USB port the solution would be to plug it into another one. If it worked, dead port on MMini. If it didn't, bad WD.
Not a highly vexing problem -- 90% of what is on there is also on my iPod (which has a larger hard drive than my computer does -- I got one of the BIG, old 160GB iPods.) I had been translating some of my DVDs to iPod format so I could watch them on the go. One batch is on that external drive but not the other one, but they are on my iPod and I have the original DVDs. Everything else is duplicated on both external drives.
Basically I'm just picking your (you, Lis, anyone else who knows) brains to see if I need to spend $60 for a replacement.

<<< for both of you!