Reformat a Partitioned Mac OS Extended External Disk?
This is an example of a procedure to erase an external disk containing a single encrypted partition.


Open the Disk Utility application and highlight the "Logical Volume
Group" representing your external disk. An example is shown below.
(Hint: click on image for a better view.)


Next, click the Info icon to get the pop up window. Note, in the
example shown below, the "Physical Backing" is disk1s2. This means
the physical disk number is 1. The "s2" suffix can be ignored. Your
numbers may be different. 


Next, highlight the "Encrypted Logical Partition" as shown in the
example below.


Again, click the Info icon to get a pop up window. Note the "Disk
Identifier". In the example shown below, the value is disk2. Your
value may be different.


Close the Disk Utility and open the Terminal application. Enter the
following commands. If your numbers differed from mine, make the
appropriate substitutions. If prompted, enter your login password.

diskutil  unmountDisk  /dev/disk2
sudo  gpt  destroy  /dev/disk1
sudo  gpt  create  -f  /dev/disk1


When I entered these commands, I saw the following in the Terminal
window.

Steelhead:~ davidanderson$ diskutil  unmountDisk  /dev/disk2
Unmount of all volumes on disk2 was successful
Steelhead:~ davidanderson$ sudo  gpt  destroy  /dev/disk1
Password:
Steelhead:~ davidanderson$ sudo  gpt  create  -f  /dev/disk1
Steelhead:~ davidanderson$ 


The distutil command  must successfully unmount the disk in order
for the gpt commands to work. If the disk will not unmount, then
some window needs to be closed. Do not proceed with the gpt
commands, if you can not unmount the disk.
If successful, you should see a result similar to what is shown
below. The result shown is a 4GB flash drive with no partitions. You
can either add a partition or repartition the entire drive.