Toshiba 2TB External Drive Stopped Being Recognised
Please quit Disk Utility if it is open and launch Terminal in the folder Applications/Utilities/ with your external drive connected
Enter the following command at the Terminal prompt:

defaults write com.apple.DiskUtility DUDebugMenuEnabled 1

Press Enter
Quit Terminal
Start Disk Utility and enable 'Show every Partition' in the Debug menu
After a few seconds all partitions should be visible
Mount the partition 'NameOfTheMainPartitionExternalHD' if it is present and greyed out with the blue button Mount. The partition should now be visible on your desktop.



In the above example the external drive is an SD card and the usually visible partition of the external drive is gDisk. Don't touch disk1s1 (Lion) or EFI (Mavericks)
Please check the format of the partition


If the format is 'Mac OS Extended' you might check the drive for inconsistencies.