Macintosh HD and Macintosh HD Data?

That's normal. The Mac HD - Data volume is where your files and apps are kept and you have access to them just like the older system volumes.

The Macintosh HD volume is where the system and system support files are kept and the user has no access to them. It's part of the new security that Catalina brings to the table.

What version of Catalina are you running? I have Version 10.15 (19A602) and only the -Data volume is shown in the Finder or in the Storage section of About this Mac but both are shown in Disk Utility.

Even if you do a fresh install with Catalina, you will get two volumes and one will be the Data volume. It contains everything except your OS/system files. So, YES, you need that - do not delete. Those two volumes are a new Catalina feature for security reasons and they are part of your Mad HD, only visible in Disk Utility.

Additionally, it is an inherently bad idea to partition your hard drive to have a backup partition on the same disk. If and when your drive fails, it will also wipe out your backup. You should always have your backup on a separate drive.

Should two drives appear on my desktop or not? If they are sharing the same hard-drive space, but are each showing up with two drive spaces that supposedly share the same space what is the point other than to confuse? Why did they only show up one the reinstall? This is very confusing.

What is going on here?

Catalina changed the filing system; your hard drive is still one drive, but it now has two volumes: Mac HD which is read only and contains the OS and system files (and cannot be changed so it cannot be accessed by malware, etc) and the Mac HD - Data which contains the apps and your files and which is read/write so you can make changes.

As mentioned, only one should show on the desktop.

If you do a reinstall correctly - meaning you first erase the entire drive - then only one drive should show on your desktop. The only place to show two volumes would be Disk utility.

And, FYI, we are all other users here and do not work for Apple, so we cannot send anything "to the appropriate people:". You will need to contact Apple for this.

You never really had a problem in the first place. Disk Utility should show Both an HD and a Hd Data. That is normal and required for Catalina. When you tried to change back to Mojave, you had already changed the file structure on your drive which is probably why you couldn't easily get back to not having the 2nd partition. This is why you probably had to re-format the disk. Tell me, did you reformat in Mac OS Extended or APFS.

If you delete HD - Data all of your non-system content will be deleted. As explained earlier in this thread Catalina splits the system disk into a write-protected region that contains system data and a “normal” region for everything else. If you change the view to see containers this will become more apparent. Your HD 2 was not affected by the update to Catalina.

Both Macintosh HD and Macintosh HD should NOT be showing up in the Finder, or even in About this Mac. They should be combined as only “Macintosh HD”. As said, only in Disk Utility should both volumes be seen.

Just make sure, when erasing the disk, to select View > Show All Devices, and erase the main HDD/SSD, as APFS, as opposed to just removing the volumes.

Also, instead of “Restoring Time Machine Backup” from recovery, it’s best, once re-formatted to “Reinstall Mac OS”, then, if need be, migrate the data during set up assistant, or set up as new, and move over the user data to the fresh user.

Ive seen some weird anomalies with Catalina since the read only/read/write volume split, it’s an awesome security concept, but can see if data is restored, or even the disk, reformatted the old way, how issues can arise, and can be kind of confusing.

I was not referring to a folder, but a volume. As mentioned, Catalina has two volumes. If you reinstall Catalina without erasing the Data volume, the reinstall may create a problem because it will create a duplicate.

I am sorry you had to pay someone. If that was an authorized service provider, do go back and ask them to erase before reinstalling. You need to have a backup of your files first.

We try to answer questions. I was not condescending, simply trying to convey facts. If you post screenshots of your Disk Utility windows showing all the volumes we can possibly help you further. It is difficult to help long distance without being able to see your screen.

Hey everyone! I finally got the answer I have been looking for for weeks! I can finally resume my normal work. Basically what happened is that once you upgrade to catalina, it partitions your hard drive to be a “ghost copy” meaning that it says Mac HD and Mac HD Data. When you go to downgrade from Catalina (for my case because most apps wouldn’t work) it’s basically keeping it as a partition instead of deleting that ghost image. For me I was unable to delete anything, even unmounting that partition. The way you are able to fix this is by going into recovery mode, unmounting the hard drive partition, and then erasing it. Problem solved, I now have more than 130gb free!

Yes, that is true, but if you just delete Mac HD Data and don’t reformat the drive all of the space it occupied becomes unusable.

The Data volume is a Volume in an APFS Container which means it doesn't have any dedicated storage of its own. Deleting the volume will return every bit of storage it used because the storage is shared amongst all volumes in the same container.

It is not a disk or a partition or anything similar.

People need to get educated about both Catalina and APFS, especially APFS. With APFS the term ‘partitioning’ no longer applies. APFS (Apple File System) has replaced HFS as the default. APFS creates what is called a ‘container’ which ‘contains’ the entire hard drive it is installed on. The user then creates ‘volumes’ within the ‘container’. The volumes appear to have the same storage space when, in fact, they are actually sharing the same space on drive. That’s how APFS works and it’s totally different than the previous file system HFS/HFS+. Apple now recommends creating an APFS volume instead of partitioning (although you still can if you want to). Creating and removing volumes on an APFS formatted drive is much easier and faster than adjusting partitions. When you remove an APFS volume its contents are erased and that space is immediately available to any volumes remaining in the APFS container.

So no, Apple did not totally ‘screw up’. People need to educate themselves how both Catalina and APFS work. Thinking as if everything is the same is the best way to get into real trouble.

Same scenario here, did a time machine backup, disconnected TM drive, then upgraded from Mojave to Catalina. Tried it, didn't like it, so reconnected TM drive and used normal Recovery process to go back to Mojave. The machine worked ok but I had the extra Data drive apparently with 230Gb of info on it. Apple support recommended completely erase drive then restore from TM backup. I could have chosen this path and wasted another 4 hours, and it was an option as a last resort... HOWEVER, using info from this thread I was able to fix the problem in about 5 minutes, without even using Safe Boot.

First, I typed "Startup" into the finder search field.This opened System preferences and confirmed my actual startup disk was the SSD drive, and not SSD-Data. So, open Disk Utility, select the SSD-Data drive and Unmount it, then Erase it. The system will automatically remount it after the delete, so Unmount it again, then select "Delete APFS Volume" from the menu options.

In an instant the SSD-Data drive had gone and the Gigabytes of assigned storage were re-assigned to the one and only Mojave startup drive. Job done.

Catalina is just too big a jump for me, too many apps wouldn't work and the cost of upgrading from Adobe CS6, Office 2011is not viable for me. Worst of all "Freecell" doesn't work! Sorry, Apple, Mojave will be my last OS.

Hope this info helps others stuck with 2 drives after reverting to Mojave from Catalina.

Yes. There is no problem. Apple just designed the new operating system to separate your data on one disk, and the operating system on another. It can be left as it is and everything should work fine.

If you are saying you want to go back to Mojave and restore your drives to their previous state, then here are the steps I took to fix the issue, copied from elsewhere in this thread, hope it helps.

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NOTE WELL, THIS IS EXTREMELY IMPORTANT.

I TOOK THESE STEPS AFTER RESTORING MOJAVE FROM MY TIME MACHINE BACKUP.

DO NOT DELETE THE DATA DRIVE IF YOU STILL HAVE CATALINA INSTALLED. YOU HAVE BEEN WARNED!

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"Apple support recommended I completely erase the drive and then restore from TM backup. I could have chosen this path and wasted another 4 hours, and it was an option as a last resort... HOWEVER, using info from this thread I was able to fix the problem in about 5 minutes, without even using Safe Boot.

First, I typed "Startup" into the finder search field.This opened System preferences and confirmed my actual startup disk was the SSD drive, and not SSD-Data. So, open Disk Utility, select the SSD-Data drive and Unmount it, then Erase it. The system will automatically remount it after the delete, so Unmount it again, then select "Delete APFS Volume" from the menu options.

In an instant the SSD-Data drive had gone and the Gigabytes of assigned storage were re-assigned to the one and only Mojave startup drive. Job done. "

This thread discusses the problem, what causes it, and the solution—>Two Volumes Named "Macintosh HD - Dat… - Apple Community

If you install Catalina on the drive the installer will partition it correctly. And if you then restore a Time Machine backup to the drive it should put it in the data partition.