Disk usage by unknown volume (Macintosh HD - Data)

Question

Level 1

8 points

Disk usage by unknown volume (Macintosh HD - Data)

Hi,

Could anyone please explain how the allocation of space between different 'Macintosh HD' is done? I am using a 2018 Macbook Pro, 256GB, standard system settings, latest OS.

In 'About this mac' > storage, I get the picture that 'Macintosh HD' has plenty of space available. This as opposed to 'Macintosh HD - Data1' and 'Macintosh HD - Data2, which seem to have less (and for some reason always have the same space available):

I have cleaned about 30-40GB of files, after which 'Macintosh HD' showed more space, but the 'Data1' and 'Data2' remained unaffected.

What is even more absurd, is that these 'Data1' and 'Data2' drives 5 minutes ago had only 19GB of room available. Now, I have pasted 20GB of movies from my external HDD into my documents (on the Mac HDD), and for some reason, the 'Data' disks have increased space available from 19GB to now 50GB.

This is not a matter of mere indication, but has operational consequences. 5 minutes ago, I could not import my 20GB movies into iMovie as there was 'no space available'. Even though, as I said, I just removed about 30-40GB of data. Now, after actually having added more files (which should have reduced available space and not the other way round...), it works perfectly fine.

This is the setup in 'Disk Utility':

As you can see, here there is even a third drive 'Macintosh HD - Data', which is not listed in the former window. It seems to occupy 182GB, which does not match the 142GB indicated in the first window. Does anyone understand how this works?

I'd be very grateful for a clarification and suggested approach how to be able to use my free space.

Best regards,

Bart

MacBook Pro 15”, macOS 10.15

Posted on Sep 14, 2020 10:13 AM

Reply

Answer 1

Best reply

HWTech

Level 9

56,882 points

Sep 15, 2020 9:58 AM in response to chopsbbq

Apple has some issues displaying the correct information in their storage management areas. Usually the most accurate information is by using "Get Info" on the volume.

The new APFS file system has some new features and behaviors that can affect the amount of storage available. For one thing when an item is duplicated in the Finder another copy is not created. So if a duplicated item is deleted only the link is deleted and the data still remains because there were "two" copies on the system.

Also the APFS file system utilizes file system snapshots that are used for making temporary copies/backups of the whole file system when updating macOS or installing apps. Plus many backup apps including Time Machine utilize this new feature. These APFS snapshot can retain data until these snapshots are deleted which should occur in a week or after a Time Machine completes transferring the backup snapshot to the external backup drive. If you try to delete large amounts of data and space is not released, then an APFS snapshot may hold a copy of that data. The space will free up once the snapshot has been deleted. Unfortunately Apple does not really tell users the details and effects of these new features/changes.

Without knowing for sure how these extra "Data" volumes were created it is hard to say whether the manner of their creation is part of the reason you are having these issues. I think it is best to straighten things out now as it will only become more difficult later on. Backup your data so you don't lose it and perform a clean install of macOS or a clean restore from the backup. Doing so should resolve the storage discrepancies too.

Reply

Answer 2

HWTech

Level 9

56,882 points

Sep 14, 2020 7:27 PM in response to chopsbbq

While Catalina now separates out the macOS system files to a read-only volume "Macintosh HD" and the user data on the writable volume "Macintosh HD - Data" in the Finder macOS will only show "Macintosh HD" so the drive appears the same as in previous versions of macOS. Personally I think hiding this is a mistake since other areas in macOS users will see the two volumes and not understand what is going on.

Usually people have multiple "Macintosh HD - Data" volumes because they have reinstalled macOS incorrectly. Regardless of how you acquired these extra "Macintosh HD - Data" volumes the easiest way to correct the problem is by making sure you have a good backup of your data, then perform a clean install of macOS by first erasing the whole physical drive before reinstalling macOS or restoring from a backup. Use these Apple instructions for erasing the whole physical drive:

https://support.apple.com/en-us/HT208496

Reply

Answer 3

chopsbbq Author

Level 1

8 points

Sep 15, 2020 2:31 AM in response to HWTech

Thank you for pointing out the difference between 'Macintosh HD' and 'Macintosh HD - Data' and that having two 'Data' disks is an anomaly.

I am fine with there being two 'Data' disks, and would at any cost try to avoid erasing my data.

Any idea as to why cleaning up files on my laptop does not result in more space being available? And how to explain the discrepancy between 'About this mac > storage' indicating 142GB free space (which seems the most realistic), while 'Data' would only have 50GB available? It cannot be the system files that you point out, as these would be merely about 11GB.

Reply

Answer 4

chopsbbq Author

Level 1

8 points

Sep 16, 2020 12:46 PM in response to HWTech

Seems to work, and this backup-restore goes quite smoothly. Thanks a lot :-)

Reply