I recently had to erase a Time Machine backup drive that had become corrupted. The drive was refusing to show when connected and would force the Mac’s Disk Utility to hang indefinitely on the ‘loading disks’ message. The disk was visible in terminal, but attempting to erase or repartition the disk both failed as the disk wouldn’t unmount, displaying error 69888. Trawling through the internet I eventually stumbled across the solution.

The solution is to kill the FSCK process. FSCK (filesystem check) is a unix disk repair utility. You can read more Here. Open a terminal (Applications > Utilities) and run the following command:
ps -ax | grep fsck
. In the resulting line make note of the number proceeding the question mark, and run the command
sudo kill {number}
Example:
sudo kill 1221

Since the introduction of Mac OS X El Capitan, the graphical Disk Utility has been stripped of most of its features. Advanced users may prefer to perform disk-related commands from the Terminal – though be warned, there are no confirmation messages here and it’s easy to make a mistake. You can run the diskutil command with no arguments for a complete list of its functions, or the Instructional have published some excellent guides which you can find Here