How convert external SSD (with macOS system volume on it) from HFS to APFS?

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Level 1

13 points

How convert external SSD (with macOS system volume on it) from HFS to APFS?

Hi all,

I'm using my SSD drive that I pulled from my old iMac (High Sierra) as an external drive to my new Mini (Catalina). The drive is formatted as HFS and I would like to convert it to APFS without losing my data, because I don't have another drive to transfer all my data to and from after the conversion.

I've read various articles and posts about how to convert without losing data, but I'm hitting a snag. The procedure I'm using is:

unmount the external drive
Start Disk Util
Select the single volume that I have defined on the external drive
Right-click on it and select "Convert to APFS..."

I get the following error:

Converting “DataDrive” to APFS

Converting HFS Volume to an APFS Container which will export one APFS Volume

Exiting conversion operations with error code -69546

Only the Installer may convert a macOS system volume to APFS.

Operation failed…

The external drive was the primary drive on the iMac which was running High Sierra, so it has a fully functioning macOS system volume on it, which is triggering the failure. Short of wiping the disk clean (since I can't get everything off and then back on later), are there files/directories that I can just remove from the external drive so that it no longer appears as a macOS system volume? I was planning on manually deleting the old system stuff anyway, like:

/Applications
/Library
/System
/Volumes
/opt
/var
/etc

What I'm wondering is if there's some specific file/directory that identifies the drive as a macOS system volume?

Any hints or tips, would be much appreciated.

Thanks!

Posted on Sep 3, 2020 4:47 PM

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Answer 1

MartinR

Level 7

24,420 points

Sep 4, 2020 8:47 PM in response to barretto94

It has been reported that Disk Utility can convert an HFS+ drive to APFS. That's somewhat true. My understanding of it is that you could do the "after the fact" conversion in High Sierra but I'm not so sure the capability remains in Catalina; second, the conversion has to be your boot drive and third you can only do it from Disk Utility in Recovery Mode. I don't think that your now-external SSD meets those conditions.

But the big thing is that if you have files that you need or want to preserve, don't convert or reformat the drive until you have already backed up the data elsewhere. Don't take chances with data you want or need to keep. Another external drive will cost you way less than the potential loss of your data.

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Answer 2

barretto94 Author

Level 1

13 points

Sep 4, 2020 7:55 AM in response to BDAqua

It was not used as a Time Machine backup drive, it was the drive in the iMac.

There's actually quite a few reasons why you'd want to use APFS as opposed to HFS+ on SSD devices:

Increases performance by eliminating the need to write changes twice compared to HFS+ Journaled file system
More efficient management of storage (i.e., additional free space)
Uses checksums for data integrity of metadata
Metadata corruption prevention due to creating new records instead of overwriting existing ones, which can become corrupt due to system crashes
Allows for clones or multiple copies of the same file, with only changes stored as deltas, which reduces storage space when making revisions or copying files
Can create point-in-time snapshots
Full-disk encryption with single or multi-key encryption for added security

The main advantage I want to use is the removal of the extra write operation which my current HFS+ Journaling system does right now. That will increase performance, extend the lifespan of the SSD, as well as allow it to operate better when the drive is near capacity.

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Answer 3

HWTech

Level 9

69,338 points

Sep 5, 2020 1:12 PM in response to barretto94

Keep in mind that APFS is still considered a new file system and it tends to become corrupted a lot where Disk Utility is unable to repair the damage. Since there are no third party utilities that can repair APFS this means you will need to erase the whole drive again. Apple has not released the necessary APFS documentation so that third party utilities can repair APFS. Personally I would never use APFS if there is a choice since I've seen too many issues with APFS on our organizations Macs.

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