Quote, Originally Posted by Spheric Harlot
Move the hard drive itself to a different enclosure and see if it works.
I had two Seagate drives (just the disks, not the enclosures) of similar vintage die on me within days of each other, and a friend had a production drive that equally failed to mount.
In all these cases, the hardware seemed fine; the drive spun up and power was okay — it merely refused to mount.
Turned out to be a serial firmware problem that many Seagate drives from that era run into eventually. No options other than a data recovery service (in my friend's case, Seagate actually recovered his data for free).
That's actually an excellent suggestion. I had a 4 year old 1 TB Seagate Drive inside a former MacPro, and the machine refused to boot. I tried everything to even "see" the drive (specifically, Target Disk Mode), but nothing worked. Turns out something must have happened with the mother board (only an educated guess). So, I took the hard drive out (along with the DVD drive; I could not even boot from it), placed both of them in separate external enclosures, and they worked fine. (I subsequently purchased a Mac Mini, and both devices are connected to it, via USB).
I'm not sure if this will help, but maybe worth a try (especially if you can save the data). I have an external Seagate 500 gig drive (purchased about 5 years ago, with a 7200 rpm Seagate drive inside), and it has 2 Firewire 800 ports ( I am using one of them for the connection to my Mac Mini), and 1 USB. If your drive has such multiple ways of connecting it to your machine, you might want to try another port on the drive (and/or another port on your machine).
Again, not sure if this will do any good, but it's something simple to try. Hopefully, there is nothing wrong with the ports on your machine.