iMac Internal Hard Drive Not Visible, Won't Mount; Currently running off External Drive My iMac (27-inch, mid-2010) crashed and refused to reboot. I was able to reboot it from an external drive and attempted to erase and resurrect the internal drive; after several attempts, the drive ceased to be visible to Disk Utility and am afraid it may be physically defective. Is there a way of mounting the disk and confirming its status?
It does sound like your hard drive has failed.

Have you tried booting up and using Apple Hardware Test? To do this hold the D key down while booting up. 

If AHT doesn't load, you can also trying running it from the internet by holding down the Option D keys while booting up.

Give this a try and let us know how you go.