iMac: Internal drive won't mount

Acutally can't find it at all. Took the machine to the Apple store and they found the drive and it diagnosed fine. They suggested getting DiskWarrior to save the data and possibly resusitate the OS.

Downloaded and found the drive in Target mode but couldn't repair it. When the DiskWarrior CD arrives I booted to DW but now I can't find the internal drive.

Any suggestions?

If you could not repair the drive and it is now unavailable to Disk Warrior it may have simply failed. Drives do.

Your internal drive is probably either defective or intermittent and should be changed out. It could also be a defective SATA connector although on an iMac that would be unusual. If the machine is still under warranty or extended Apple care, take it back to the Apple store and have them change it out. If not under warranty, you might want to do the job yourself although it's a rather difficult task.

Added a new 1TB drive and upped the memory to 4GB while I was at it. Everything running fine. Now I just need to try and access the old disk so I can migrate my data and apps.

Well you have learned one thing. Get an external case and hard drive and backup backup backup!