APFS File System On External Drives

Now that I have been using the APFS version of High Sierra on my internal iMac SSD for a week or so, I decided to format all my external drives to APFS as well. I tried this the first day in APFS High Sierra and it would not allow me to do the format, but things have changed.

APFS & Disk Utility

After moving my Photos Library back on to my internal SSD I decided my next step was to format all my external USB drives (I have several of them) to APFS. Here is how that works.

Open the new Disk Utility app in High Sierra:

When you first open Disk Utility you will not see all of this information in the sidebar. You will just see a list of the “logical drives”. To expand that list, just click on the “View” menu in the upper left corner and choose “Show All Devices”.

To change an external drive into the APFS format you start by right-clicking on the logical drive name and choosing “Convert to APFS” from the drop down menu:

Disk Utility will then warn you that the drive cannot be used with older macOS versions if converted:

When you click “Convert”, Disk Utility does the conversion:

This took a couple of minutes for my machine, YMMV. Once the conversion is finished you get this completion window:

If you do a Get Info on the drive in the Finder you will see it is APFS:

I am pretty sure I will not need to read these drives with older Mac OS’s. That is why I went ahead and did the conversions.

One Caveat

There is one caveat here. When I first tried this in High Sierra it would not allow the conversion, as previously mentioned. After converting all my external drives, I tried to convert my external Clone drive of my iMac. I use Carbon Copy Cloner to make the clone once a week. Here is what I got when I attempted the conversion on this drive:

If you look closely at the drop down menu, the “Convert to APFS” command is grayed out. I even booted into the Clone thinking that may make a difference, but no joy.

So, why did the other drives work and not the Clone drive? Here is my theory. I think after you interact with external HFS+ drives with your main APFS system it will allow you to upgrade them to APFS. I guess it has to be taught these drives are there or something. I am not really sure, but it does work now.

Here is my theory on the Clone drive. Carbon Copy Cloner sets up the Clone drive to be bootable. It is a bootable HFS+ external drive. I think because it is bootable, Disk Utility will not allow it to be upgraded to APFS. If I ever wish to get my Clone drive to APFS, I think I will have to Erase and format it to APFS from scratch, then put a clone onto it. Another project for another time.


My High Sierra install is running very well. There are a few issues which I am sure will be fixed in High Sierra ver. 10.13.1 which is already out in Beta format. If you don’t feel comfortable formatting your external drives to APFS, there is no harm in waiting a while. But, if you are game, it is pretty easy to do. My APFS formatted external drives are all working fine by the way.