Was this reformatted specifically for Mac?
Watching these threads over time, I would say it is just as common for the external enclosure to fail. If this is what has happened removing the drive and putting it into a different enclosure will cure the problem.
If you have not as yet done a reboot on the computer that would be my first step. Also look for the drive in Disk Utility. You may be able to select it even though it is greyed out, then click on the mount tool.
When you unmounted did you wait at least 10 seconds before disconnecting? In theory it should be safe the instant the icon disappears from the desktop, but I have seen that not properly disconnected dialog box far too many times when people were in a big hurry.
Watching these threads over time, I would say it is just as common for the external enclosure to fail. If this is what has happened removing the drive and putting it into a different enclosure will cure the problem.
If you have not as yet done a reboot on the computer that would be my first step. Also look for the drive in Disk Utility. You may be able to select it even though it is greyed out, then click on the mount tool.
When you unmounted did you wait at least 10 seconds before disconnecting? In theory it should be safe the instant the icon disappears from the desktop, but I have seen that not properly disconnected dialog box far too many times when people were in a big hurry.