three hard drives - 80GB ide drive; 160GB SATA drive and a 500GB SATA drive. All 3 were working fine then --->

I shut down the pc, and some time later tried to start it up again. It went into a long wait trying to identify the sata drives; It finally identified the 160GB drive but not the 500gb drive - it was totally inaccessible.

Hooked up the 500gb drive to a usb port using an ultra ide/sata cable adaptor. The drive was recognized; every method I used to look at it (gparted, testdisk, fdisk)
Showed it to be a 2TB drive.

Where is that information comming from --- is it somthing stored on the hard drive, or is it comming from corrupted firmware on the hard drive??

Fdisk reports that there is no partition table; I tried to write an empty one and to change the number of cylinders back to a value close to what it was; fdisk said it worked, but when I ran fdisk, again, on the drive and it looked like nothing 'took'

That 500gb drive was my backup and multimedia drive and there is some data on it I would like to recover. Are my chances any greater than a snowball in a blast furnace?

Did you try parted? There is a "recover" option that you could try to use to recover the partition table. (See man parted for details.)

That should work with the USB connection, but (if I were doing it) I'd reconnect the drive with the SATA cable, after removing (commenting out) any references to the drive in /etc/fstab so it won't be auto-mounted. If you can recover the partition table, run an fsck on the drive, and then see if you can mount it by hand.

You might also consider booting from, e.g., the SystemRescue Live CD which contains many rescue tools and, if all else fails, tools for recovering files from damaged drives.

You should also investigate why shutting down you PC bollixed up the drive. That sort of thing usually only happens when the mains fail and you don't have a UPS, or the PC's plug is pulled. And that only rarely happens. (Perhaps your SATA cable was lose or improperly connected?)

Thats where I ran gparted from; Also testdisk. I did not allow testdisk to finish --- it looked like it was going to run well over 24 hours. Thats also when I finally realized that the drive - and everything looking at it - thought it was a 2TB drive.
Thats a possibility. Those sata cables seem to me to be very insecure; it seems they come loose just looking at them.

Ran parted on the drive from a slackware system,
here is a sample:could find no 'recover' option, how about 'rescue'?

Yes, my bad. rescue is the command. ("recover" was, I think, an option in gparted or, maybe, qtparted, at one time, but I've not seen it recently.)

Did you try, for example, a rescue 0 1000K to see if a partition table could be found? (Of course, if you actually applied the commands you displayed in your last post, you will have overwritten any partition table information that was available on the drive, and the "rescue" will just find the new table information.)

IIRC SysRescue includes foremost and other file recovery tools, so you may still have a chance of retrieving your critical files.

Is it possible that the drive is really "dead";
that is the disk not even spinning?
How can I tell?

If the drive is still in the USB configuration, pick up the drive box while it's plugged in, and place your ear next to it. You should feel and hear some vibration from the fan and drive.

If it's back in your box, put your finger on the drive while it's powered on and you should be able to feel it spinning.

Note, however, that it's very unlikely that the drive would be detected by the system at all if it wasn't spinning.