Q In spite of being able to ‘see’ my 2 TB Seagate external drive in Disk Utility, its icon is greyed out, and I am unable to mount or eject it. Neither will it erase or repartition. How can I restore it to normal function?
A Shut your Mac down with the external drive still running; if this fails because the disk cannot be unmounted, force shutdown by holding the power button. Once your Mac has shut down, power the external drive down, perhaps by switching it off.
Then leave them for a minute or longer before powering up the external drive, letting it run up to speed, and only then starting up your Mac.
Open Disk Utility and see if it works again, to re-partition the drive into a single volume. If Disk Utility still cannot access the drive, you could try other tools such as TechTool Pro, or dedicated repair utilities such as Drive Genius. Sometimes reformatting a disk using a Windows system can help it mount so that you can initialise it again in Mac Extended format using Disk Utility. If nothing can access the drive properly, then sadly it has almost certainly gone west.
Updated from the original, which was first published in MacUser volume 28 issue 12, 2012.