Macintosh HD - Data: what is that?

Same thread just select All Replies, page 2. But to help now, I copied and pasted the whole reply. Print the instructions and take your time:

"Last week I upgraded to Catalina and because of Catalina problems, I restored Mojave 10.14.6 successfuly from my Time Machine backup. During the process, I too wound up with the " Mac HD - Data" in Disk Utility and my used space more than doubled. It annoyed me that Catalina could force someone to buy larger external drives for backups or worse, have to buy a new computer. I spoke to a Senior Tech at Apple this AM and my disk is back to normal. Here are the steps involved - BE CAREFUL. If unsure, leave well enough alone.

1) Back up computer (!) even though it will take a long time

2) Shut Down Mac (not restart)

3) Start Mac - immediately hold down the CMD-R keys

4) When the Apple logo appears and you see the progress bar, release the keys

5) You will see eventually the MacOS Utilities screen

6) If you cannot move the arrow, use a USB WIRED Mouse (a real possibility)

7) Select Disk Utility and then Continue

8) Select "Macintosh HD - Data" DO NOT select ERASE. Please - be careful!

9) Upper menu bar of your Mac - click Edit

10) Select "APFS Volume Macintosh HD - Data" (careful - make sure of what you select) & Click 'Delete'

11) When all finished, go to upper menu bar of your Mac and select Restart

12) After you have restarted, check Disk Utility to make sure Mac HD-Data is really gone

What caused an additional mess for me was doing the Security Update 2019-002. Apple is working on a fix to prevent the Mac from freezing when installing this Security Update as other Mac users have the same problem.

Again CAUTION! Do NOT delete "Macintosh HD". Make sure you select "Macintosh HD - Data". If feeling queasy about any of this, do NOT do it. After I had performed this process, I asked the tech to repeat everything to me and I wrote it down and reconfirmed. BTW, my disk is normal now, Mojave works great. I do not need to buy larger capacity hard drives for backups.

Good luck!"

It's perfectly normal. The disk is split in 2 volumes for security reasons.

https://arstechnica.com/gadgets/2019/10/macos-10-15-catalina-the-ars-technica-review/11/#h1

so I think I’m going to try another full wipe and restore.

Make sure you erase the entire device, not just the Macintosh HD volume.

In Disk Utility, click the View button and choose Show All Devices.

Last week I upgraded to Catalina and because of Catalina problems, I restored Mojave 10.14.6 successfuly from my Time Machine backup. During the process, I too wound up with the " Mac HD - Data" in Disk Utility and my used space more than doubled. It annoyed me that Catalina could force someone to buy larger external drives for backups or worse, have to buy a new computer. I spoke to a Senior Tech at Apple this AM and my disk is back to normal. Here are the steps involved - BE CAREFUL. If unsure, leave well enough alone.

1) Back up computer (!) even though it will take a long time

2) Shut Down Mac (not restart)

3) Start Mac - immediately hold down the CMD-R keys

4) When the Apple logo appears and you see the progress bar, release the keys

5) You will see eventually the MacOS Utilities screen

6) If you cannot move the arrow, use a USB WIRED Mouse (a real possibility)

7) Select Disk Utility and then Continue

8) Select "Macintosh HD - Data" DO NOT select ERASE. Please - be careful!

9) Upper menu bar of your Mac - click Edit

10) Select "APFS Volume Macintosh HD - Data" (careful - make sure of what you select) & Click 'Delete'

11) When all finished, go to upper menu bar of your Mac and select Restart

12) After you have restarted, check Disk Utility to make sure Mac HD-Data is really gone

What caused an additional mess for me was doing the Security Update 2019-002. Apple is working on a fix to prevent the Mac from freezing when installing this Security Update as other Mac users have the same problem.

Again CAUTION! Do NOT delete "Macintosh HD". Make sure you select "Macintosh HD - Data". If feeling queasy about any of this, do NOT do it. After I had performed this process, I asked the tech to repeat everything to me and I wrote it down and reconfirmed. BTW, my disk is normal now, Mojave works great. I do not need to buy larger capacity hard drives for backups.

Good luck!

If anyone wants to erase the disk(s) and make a clean install of macOS Catalina, follow this:

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

Pay attention to what must be done with "Macintosh HD - Data" and what with "Macintosh HD".

Hope this helps those who have my same question.

TM

Leave it as is.

The Macintosh HD - Data is the drive/volume that you'll be working from, housing your applications, home folder, documents, etc

Why the drive is full is quite a quandary these days.

Since the two Volumes are part of the same APFS Container, they share the same physical storage space on the drive. If one is full, the other is full.

Now, you can set Quotas on a Volume in a Container, but I doubt Apple would have done that in this case. If one Volume hit its quota, it would report full whereas the others might not.

I've read that reverting to an earlier OS without erasing the hard drive is the cause of the sudden two Mac HD - Data volumes. If, while in recovery, you erase the entire disk (top entry), that will not happen.

As far as I know, you can use Carbon Copy Cloner to do a bootable clone in the exact manner you did pre-Cataline and it will take care of the two Mac HD/Data drives.

The relevance, Terrance, from my perspective, is that I had a functioning work computer and after the installation of Catalina I unexpectedly had this extra hard drive icon on my desk top, and I was receiving warning messages that they were both full. BEFORE I did this I had one Macintosh HD icon that had about 520gb of free space (It's a 960gb SSD).

As I hadn't heard of the Macintosh HD – data disk I was confused as to what was going on and why.

And worried that I was about to lose everything.

So Terrence, if you can put your arrogance to one side and acknowledge that some other people are experiencing these issues and may not be as wise as you this would be helpful.

Some mixed issues here I think. I reverted to Mojave due to incompatibility issues. I used a Mojave TM backup, wiped my Mac, etc and these drives are still both there. Also, my Mac is using each of them for user data, seemingly at its discretion. All I can think of atm is the restore did not revert to single partition of Mojave, and kept some of both. This shouldn’t be possible but it’s what happened, so I think I’m going to try another full wipe and restore.

And if I want to erase the disk and make a clean install? What do I do with these partitions?

If you erase the device, it will remove the partitions.

I am getting disk full messages on both the HD and the HD data discs. The data disk only haS the system and apps on it but it is telling me it’s as full as is the had disc which has the above, my non-Apple apps (creative cloud etc) and all my work on it. So I’ve got two ‘partitions’ and both of them are full. Fantastic. Back to Mojave for me.

Believe it or not, I'm quite aware that this forum is full of people having issues. Indeed, these issues are the reason people come here. The original poster asked for an explanation. It was given. You then hijacked that simple process with a complaint about an unrelated problem you're having. Not sure what your definition of arrogance is, but mine includes entitlement.