MacOS – How to convert macOS Extended Journaled to APFS

This is an example of a procedure to erase an external disk containing a single encrypted partition.

1. 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.)

   

2. 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.

   

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

   

4. 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.

   

5. 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.

6. 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.

   

Awfully hard to test our software on a new file system if I can't convert the boot drive to the new file system

Since you have space to spare: convert an empty partition to APFS, then install 10.13 on the APFS partition. Or use SuperDuper or somesuch to copy your existing 10.13 installation to the new partition.