Say you open Disk Utilities and you want to add a partition to a hard drive or external usb. maybe you did it a few times and all the sudden it disappeared??

Just Happened to me! You can fix this this in terminal but there is an easier way.

First the root cause is the Partition Table Schema has gone awry. It needs to be GUID.

To get it back, lets click the toolbar view > show all devices. This will show the parents that were invisible prior

Now you will need to erase the main container and reformat so backup anything first and choose these settings (Mac OS Journaled, GUID)

After that funny enough you can create partition of any format FAT exFAT etc.

Cheers!