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Merging two disks

Hi, I hope someone here could help me. I am using Mac Book Pro 13 (Intel, 2017). When I got my Mac, someone advised I create 2 'partitions' so I divided my disk in 2 (now visible in Startup Disk as 2 separate startup disks). I updated the OS on one of the disks to Big Sur 11.2.3 (20D91) while the other one is still on Catalina 10.15.7. I am completely lost now with what is where, I managed to make Time Machine backups of both disks and I would like to merge them in one so everything is in one space. How can I do it?

MacBook Pro 13″, macOS 10.15

Posted on Aug 7, 2021 1:36 PM

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Question marked as Top-ranking reply

Posted on Aug 7, 2021 6:07 PM

That was terrible advice instructing someone to install two versions of macOS to the same drive as that usually leads to problems including getting confused where your data is stored. If you need access to an older or newer version of macOS or just want to try out the newer version before upgrading your main drive, then installing an OS to an external hard drive is a much safer option as it is less likely to impact your main macOS boot drive.


The simplest method moving forward is probably to boot into the OS you want to remove. Manually transfer your files from within your home user folder(s) to an external drive which is partitioned and formatted as GUID partition and MacOS Extended (Journaled). If you have multiple user accounts on that OS, then make sure you transfer the data from all those user accounts.


Once you have transferred all the important files from all the user accounts on the OS you want to delete, then you reboot the Mac into the other OS which you want to keep. Now you will need to delete the APFS volumes and/or APFS Container associated with the other OS. In theory macOS should not allow you to delete any APFS volumes or Containers for the OS you are booted into.


Another more drastic option (perhaps simpler option) is to perform a clean install of macOS by erasing the entire physical drive before reinstalling macOS. You can then restore or migrate from a TM backup as long as the OS for the TM backup is for the same or older OS. Again make sure to have a good backup of both operating systems or make sure you have transferred all your important files to an external drive as previously mentioned.


Make sure to disconnect your external drives and backup media before attempting to delete any APFS volumes/Containers, or before erasing any drives. You don't want to accidentally delete or damage your backups or externally stored data.


After you get back to a single OS you will need to figure out which files on the external drive from the now deleted OS are unique and which of those files may be newer versions of the ones on your current user account. There is no easy way to avoid this step. It will require you to look at each file.


In the future if you ever use more than one OS on a computer or if you have multiple computers, then make sure to store your files in a shared location so that you have just a single location where you make modifications to those files. Having them stored on an external drive or a network share are two options instead of storing them within the home user folder of any one system. A cloud service can allow you to sync files between multiple devices, but some of them may affect system performance plus you risk someone gaining access to anything stored in the cloud. No matter how you do this you will want to make sure to have a good backup of your computer and any external media (includes external drives, network drives/shares, or items stored/synced in the cloud).


I find it is best to keeps things as simple as possible as it makes management much easier and it is less likely for mistakes to happen.


No matter what you should make sure to have a good backup of both operating systems just in case you make a mistake.


You should always have frequent and regular backups.


Similar questions

7 replies
Question marked as Top-ranking reply

Aug 7, 2021 6:07 PM in response to karolinakaja

That was terrible advice instructing someone to install two versions of macOS to the same drive as that usually leads to problems including getting confused where your data is stored. If you need access to an older or newer version of macOS or just want to try out the newer version before upgrading your main drive, then installing an OS to an external hard drive is a much safer option as it is less likely to impact your main macOS boot drive.


The simplest method moving forward is probably to boot into the OS you want to remove. Manually transfer your files from within your home user folder(s) to an external drive which is partitioned and formatted as GUID partition and MacOS Extended (Journaled). If you have multiple user accounts on that OS, then make sure you transfer the data from all those user accounts.


Once you have transferred all the important files from all the user accounts on the OS you want to delete, then you reboot the Mac into the other OS which you want to keep. Now you will need to delete the APFS volumes and/or APFS Container associated with the other OS. In theory macOS should not allow you to delete any APFS volumes or Containers for the OS you are booted into.


Another more drastic option (perhaps simpler option) is to perform a clean install of macOS by erasing the entire physical drive before reinstalling macOS. You can then restore or migrate from a TM backup as long as the OS for the TM backup is for the same or older OS. Again make sure to have a good backup of both operating systems or make sure you have transferred all your important files to an external drive as previously mentioned.


Make sure to disconnect your external drives and backup media before attempting to delete any APFS volumes/Containers, or before erasing any drives. You don't want to accidentally delete or damage your backups or externally stored data.


After you get back to a single OS you will need to figure out which files on the external drive from the now deleted OS are unique and which of those files may be newer versions of the ones on your current user account. There is no easy way to avoid this step. It will require you to look at each file.


In the future if you ever use more than one OS on a computer or if you have multiple computers, then make sure to store your files in a shared location so that you have just a single location where you make modifications to those files. Having them stored on an external drive or a network share are two options instead of storing them within the home user folder of any one system. A cloud service can allow you to sync files between multiple devices, but some of them may affect system performance plus you risk someone gaining access to anything stored in the cloud. No matter how you do this you will want to make sure to have a good backup of your computer and any external media (includes external drives, network drives/shares, or items stored/synced in the cloud).


I find it is best to keeps things as simple as possible as it makes management much easier and it is less likely for mistakes to happen.


No matter what you should make sure to have a good backup of both operating systems just in case you make a mistake.


You should always have frequent and regular backups.


Aug 12, 2021 4:07 PM in response to karolinakaja

The Container that has the "Update" volume belongs to Big Sur so the other Container most likely belongs to Catalina. While booted into the OS you want to keep you should be able to use Disk Utility to delete the other Container and in theory merge the space used by the deleted Container with the remaining Container.


If you want to erase everything, then boot the macOS installer (USB or Recovery Mode) and use Disk Utility to erase the whole physical drive as GUID partition and APFS (top option). You will need to click "View" within Disk Utility and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility. You can find links to various macOS installers within this Apple article along with instructions for creating a bootable macOS USB installer:

https://support.apple.com/en-us/HT201372



Aug 8, 2021 11:06 AM in response to HWTech

Hi, thanks a lot for all the tips! I think I managed to use the more drastic option you described: I tried erasing the disks I saw. I managed to start my laptop again and it's back to High Sierra 10.13.6 (17G66) system... Plus, when I now check the Dish Utility it looks like I have a disk called Update (APFS Volume, encrypted, with around 350 GB on it) and another one called Container Disk2 (APFS container, with over 149 GB from which there is 3 not mounted - I have no idea what this means). The minus button is greyed out on the latter, there is one on the Update disk. (I attached the screen shots)

I am not sure how I should make it being one single disk. I would like to update my system to the latest version of MacOS but it seems more logic to merge the disks first. Would you know why I may have these two disks and how to proceed? Massive thanks!

Aug 9, 2021 4:52 PM in response to karolinakaja

I did not look closely enough at your screenshots earlier. It seems the "Update" volume contains your installation of macOS. The Disk Utility GUI has been too overly simplified by Apple making the app extremely difficult to use and to understand the disk layout. You may first need to click "View" within Disk Utility and select "Show All Devices" so that the physical drive and hidden Container appear on the left pane of Disk Utility in order to get a slightly better view. I've seen bugs in how Disk Utility displays things.


It is much better to use the command line to view the disk layout since it shows everything as it is unlike the Disk Utility GUI app which tends to mix things up. Launch the Terminal app and run the following command (you will need to press the "Return" key at the end of the line to execute the command):

diskutil  list  internal


Post either a screenshot or the complete output here using the "Additional Text" icon which looks like a piece of paper.


Merging two disks

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