Merge double APFS Catalina volumes on Mojave after downgrade?

[TheSW]

I "downgraded" my 2020 MBP laptop from Catalina to Mojave by creating a bootable external drive with Mojave installed from another Mojave computer. I booted the MBP from that external drive and erased the internal MBP SSD and formatted it as APFS. Don't recall if I saw the two volumes when I kicked off the reformatting but I now have the "twin" Catalina boot volumes. One "main" volume that I can rename, the second called "Internal-Data". I've been merrily installing SW etc on the MBP without paying attention to the twin drives. At this point the main volume has ~80G used, internal ~20G. Both in the single Container on the SSD. I understand these are the result of the previous Catalina install. Is there any way to merge these two volumes? I'm not planning to "upgrade" from Mojave any time soon - preferably never but that's another thread.

 

[opeter]

I don't think you can merge with the default Apple Disk Utility, that is built into the system.
But you can try, read this thread:

How do I merge my APFS Container with my … - Apple Community

discussions.apple.com

If it wont work, maybe you try with something like Paragon HD Manager:

Paragon Hard Disk Manager for Mac | Paragon Software

Paragon Software Group delivers solutions for hard disk management, partitioning, drive backup

www.paragon-software.com

But I am not sure, so you need to test it...

 

[TheSW]

Thanks opeter. My guess is there's no "simple" way to do this. It will/would probably involving moving data to an external drive and reformatting the SSD but just where/how the data merge would occur is the hard part. Having to do it manually would be a nightmare in which case I'd just live with the twin drives. I'll take a look at the Paragon tools. Much appreciated...

 

[panjandrum]

Well, Mojave is much better behaved about all of these things than Catalina is. You *should* be able to do this:

1) Once you have your system working as you wish in Mojave, use an external drive as a full clone destination for Carbon Copy Cloner. You do not necessarily have to use the entire drive, a partition will be fine. Note that you do need to make sure this external drive has the correct map (GUID) etc. CCC is usually pretty good about warning you if things don't look right.

2) After you make that full, bootable clone, make sure it works by booting from it. Do NOT proceed to step 3 until you are absolutely sure that your clone is good!

3) Now that you are booted from the clone, do a full erase of the internal drive using Disk Utility. Don't just erase the containers. Use the "View" icon to select "Show All" and select the drive itself. Then erase it (GUID and APFS). Note that sometimes it takes several attempts to get the drive to "unmount" so you might see failures, but eventually it will work.

4) Clone back to the internal storage again using CCC.

5) Done (Well, use Startup Disk in System Preferences to set the startup disk to the Internal storage, just in case it didn't figure that out automatically.)

 

[TheSW]

That sounds like a great solution panjandrum! So essentially CCC recognizes the twin Catalina volume paradigm and internally merges the data into a single volume before writing the (single non-APFS) backup volume? Brilliant! I use CCC frequently (just installed it on the MBP earlier this morning) and am familiar with the "View" options in Disk Utility. Time for me to find another free external drive and give it a shot.

Much appreciated!

(I think I'm still going to check into the Paragon tools - looks pretty interesting)

 

[panjandrum]

That sounds like a great solution panjandrum! So essentially CCC recognizes the twin Catalina volume paradigm and internally merges the data into a single volume before writing the (single non-APFS) backup volume? Brilliant! I use CCC frequently (just installed it on the MBP earlier this morning) and am familiar with the "View" options in Disk Utility. Time for me to find another free external drive and give it a shot.

Much appreciated!

(I think I'm still going to check into the Paragon tools - looks pretty interesting)

Yes, I *think* that will probably work. I'm not sure as I've never had to migrate backwards. I long ago decided not to migrate to Catalina as it had way too many problems, and the few machines I had to move "backwards" I could just fully wipe. But CCC knows what partition schemes each OS uses and will most likely work. Once you have that system running on Mojave in the first place, I don't think it should even been using the "data" volume for anything anyway. It's probably just sitting there as wasted space. You won't know for sure until you try.

 

[TheSW]

First thing that occurred when I kicked this off is that I realized of course that CCC requires you to choose a (single) source volume - oops. I chose the "main" volume, did the backup and tried to boot from it - no luck. At the end of the backup CCC reported that it "thought" the volume would be bootable. It got to a black screen with the arrow showing but after 5 minutes no change. I'll try the other volume and see what happens. One article I read on these volumes claimed the data volume might actually be the one the OS pays attention too? I'll see. Wish I could choose a Container in CCC but apparently that's not an option. I'll update later on progress with my second attempt. Thanks again.

 

[TheSW]

No luck with the Internal-Data volume backup. CCC created the volume just fine on the external disk but it was not even an option in the StartupDisk prefs. Back to square one. Maybe the Paragon tools will offer a solution.

 

[panjandrum]

No luck with the Internal-Data volume backup. CCC created the volume just fine on the external disk but it was not even an option in the StartupDisk prefs. Back to square one. Maybe the Paragon tools will offer a solution.

Rats. I may have misunderstood where you are at in this process? It sounds to me like you are at the point where you have internal storage on which you have Mojave successfully installed and working properly? And that the problem is that your internal storage retains the 'data' volume that Catalina created? I've never seen Mojave use that volume at all. That whole split-volume wackery came into being under Catalina.

Ultimately, your best bet is to reformat the entire drive properly (see my step 3 above). If CCC won't work, you could try doing a clean-install of Mojave to an external drive, then using Migration Assistant to migrate everything from your internal storage to the external drive. Make sure that works properly. If all is good, then you should be set to erase the internal storage properly. You could just clone back using CCC, as that's easier and faster than reinstalling and Migrating again.

 

[TheSW]

Correct - Mojave is working correctly but data is split across the two volumes. Why? Probably because I wasn't paying attention when I reformatted the SSD and let it create the two volumes it saw there from the previous Catalina install - dooh!

I had an insight as I went to reformat the clone disk I used for CCC. After reformatting it (into GUID) the OS asked if I wanted to use it for my Time Machine backup. Hey! Can I create a Time Machine backup to this non-APFS disk then do a migrate to a reformatted SSD? (SSD will still needs APFS but I can be more careful during the reformat to not make the two partitions) Time Machine backup is underway now so the first step works. When done I'll experiment by creating another external Mojave boot disk and then migrate to it from the Time Machine backup and then make sure that boots.

This is basically what you suggested immediately above - just a slightly different sequence (I think).

Next report when all this is done. Probably tomorrow.

Thanks panjandrum!

 

[TheSW]

Solved

The solution is apparently buried somewhere in Time Machine or Migration Assistant.

1) Mount, erase and format an external drive #1 (ED1) as Mac OS Extended (Journaled), GUID Partition Map

2) Set ED1 as the MBP SSD Time Machine backup drive and do a complete backup to it. Leave it mounted.

3) Mount, erase and format a second external drive (ED2) as APFS, GUID with a single partition.

4) Install Mojave onto ED2. During the installation process, when asked if there's data to be migrated tell it to migrate data from a computer or backup (the still mounted ED1 Time Machine disk). This is where the first speed bump occurred.

5) Although ED1 is not formatted as APFS, the two different partitions ("main" and "internal-data") from the MBP backup appeared as options to select from as a source for the data migration to ED2. Note - it (i.e. the Time Machine disk ED1) did not appear as two mounted volumes on the computer - just a standard single Time Machine disk. At this point I feared I was back to my original problem but pressed on and decided to select the "main" partition to migrate from. Just an "educated" guess.

6) The migration took some time as it was reading/writing to an external disk. A few restarts were forced but eventually ED2 mounted as a single Mojave drive. The only question as this point was what was on it? Did the full contents of the MBP migrate or just some subset contained on the "main" partition. The best answer I can provide is that it appears everything from the MBP was on ED2. I say "appears" because although a quick check revealed "all" my recently installed apps and data were there, a comparison of data size/files between the two provided no clues. For example, sizes of each volume showed: SSD "main" volume 79.2G, "internal-data" volume 18.8G and the new ED2 clone as 88.1G. Similar inconsistencies showed up in lower level folders. From what I could see the "internal-data" drive contained some amount of system "helper" data. For example, in its' Application folder were only about 6 apps that would not run on Mojave. In fact I don't even know where they came from. Bottom line - I went with the assumption that ED2 was now effectively a clone of my MBP. Before moving on I checked SW update and the 3 Mojave updates that I'd done on the MBP did not appear in the ED2 system so I went through that update process and restarted and booted ED2.

7) While booted by ED2 I invoked Disk Utility, erased and reformatted the MBP SSD as APFS/GUID (single partition).

8) Using Carbon Copy Cloner I cloned ED2 to the MBP SSD and restarted.

9) The machine booted fine and its' what I'm writing this message on.

So, why does this work? Where does the magic merge occur? My guess is that APFS has some sort of linking mechanism between the two partitions and either Time Machine or Migration recognized those links and copied complete files? Just a total guess though.

Sorry for being so long winded but many times it's the little details of "solutions" where people can get stuck.
Hope this helps some others in the future.

A big thank you to opeter and especially panjandrum for taking time to offer advice.

Steve

 

[panjandrum]

Good to hear that you got it working.

 

[opeter]

Congratulations! Thank you for your experience and the written procedure. I believe that this information will be useful to anyone else too... 👍