Problem after installing MacOS on external disk

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Level 1

5 points

Problem after installing MacOS on external disk

I'm using a MacBook Pro 13 inch from Mid 2012. I'm running Catalina 10.15.7. I know this old old equipment but I can't afford a new Mac, and my current one has worked fine.

I had a crash a couple of months ago, got the machine running again, but got fed up with its complaints about not enough room on the internal disk so I decided to move the machine to a 2TB external disk. As far as I know the installation worked fine but...

Before this update I had four disks associated with my Mac: The internal disk is 120 GB and is called Macintosh, there is an external disk of 320 GB called PO-40595, and the 2TB disk called Bigjohn. There was a 1 TB disk for backups but I have moved that one to another (Linux) machine temporarily.

After the installation (all done according to the rules) I now have only the two external disks above, PO-40595 and Bigjohn, visible in Finder.

When I go into About this Mac, I can see the following:PO-40595, Macintosh - Data, Macintosh, Bigjohn - Data and Bigjohn. I cannot find any way to see the contents of the internal Macintosh disk. I don't understand the difference between Macintosh and Macintosh - Data which appear to be identical except for the BSD Name whatever that is. And of course I have a Bigjohn - Data and a Bigjohn.

I'd be grateful if anyone can explain the difference between a disk and its Data equivalent, and more importantly, how I can load the Macintosh disk and hopefully read and copy the contents thereof. And why does PO-40595 not have a Data equivalent?

Thanks in advance,

James

MacBook Pro 13″, macOS 10.15

Posted on Dec 20, 2025 1:56 PM

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Answer 1

HWTech

Level 9

68,750 points

Dec 20, 2025 2:42 PM in response to Deltagande

Beginning with macOS 10.15 Catalina, macOS now resides on two separate APFS volumes which are "Macintosh HD" and "Macintosh HD - Data". "Macintosh HD" is a read-only APFS volume containing only the macOS system files (aka the OS itself), while the "Data" volume holds your home user folder(s) & data along with system configuration files that need to be modified so cannot reside on the read-only system volume.

These two APFS volumes are "magically" linked together behind the scenes so they appear as one traditional "Macintosh HD" volume like older versions of macOS used. Some areas within macOS show the split nature.....Disk Utility is one of those places where the truth is seen.

There are also some other APFS volumes hidden from view even from Disk Utility.

See the following archived Apple article for details:

About the read-only system volume in macOS Catalina or later - Apple Support

Unfortunately when you boot more than one version of macOS, you will see the separate APFS volumes which may not always be still connected (depends on the version of macOS).

If you want to access the data on another Catalina installation, then you will need to access the files by accessing its related "Data" volume which will be seen under "/Volumes" when mounted. You may need to manually mount it using Disk Utility. If you have three separate drives with Catalina installed, then it may get extremely confusing which "Data" volume is which especially if it is only mounted as "Data" and not "Macintosh HD - Data", or in your case "BigJohn - Data". I highly recommend you use Disk Utility to unmount the "Data" volume you are not interested in at the moment to make it a bit easier.

Deltagande wrote:

And why does PO-40595 not have a Data equivalent?

Because it may still retain its original name "Macintosh HD - Data". Yes, you can change the base name of one of these paired volumes, but not the other because Finder and Disk Utility are utterly broken in this regard.....even multiple major versions of macOS later.

Use Disk Utility to see the related system & Data volumes paired together and make sure the each pair has the same base name. When both pairs (system & data) volumes have matched names, then you can usually use Disk Utility to change the base name of one item in the pair (or even the "Volume Group") and it should cause the base name of the other one to change to match it. Always leave the " - Data" part untouched. You may need to right-click or Control-click on the name in order to select "Rename" option.

FYI, in later versions of macOS, macOS will only show the "Data" name for the mount point of the Data volume so it can become more confusing, although some versions of macOS may be able to just show the base name as it mounts the pair as one. Apple has really made a mess of things here by not properly testing things before releasing them to the public. I has gotten better over the years, but there are still some issues in this regard.

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