Take a look at this screenshot of my MBA’s Disk Utility:
Why are there three? I know Catalina separated user data + system data, but that’s two disks. Did I miss something? Are there supposed to be 3, and if so, what is the third one?
It is even more complicated:
https://bombich.com/kb/ccc5/working-apfs-volume-groups
I am not 100% sure, if you have an issue or not.
APFS can be weird…
Maybe somebody else has more knowledge on this one… 
Is that the recovery partition?
That was my first guess, too.
But after reading the article by Bombich Software I am not so sure about that.
There should be Preboot, Recovery and VM (all of them hidden), system and data (forming a volume group, and both of them showing up in Disk Utility) and potentially another volume if somebody has created one inside of an APFS container.
I am confused.
EDIT:
I just checked my Mac Mini. Disk1 is shared by 5 volumes. Macintosh HD and Macintosh HD - Data share one container. I have the VM with 2.15 GB, two “not mounted” volumes of only 752.8 MB and free space. Those two “not mounted” volumes should be Recovery and Preboot according to the mentioned article. So, yes, the third Macintosh HD volume looks weird.
I wonder what Disk Utility is displaying when @ThatNerd clicks on the greyed-out “Macintosh HD”…
It’s not totally clear from your screenshot but it appears you have two volumes labeled “Macintosh HD - Data”. This is an artifact of reinstalling Catalina. Were you, by any chance, running the Catalina beta at any time, before installing the release version?
I reinstalled Catalina ~2 months ago. Did that create the disk?
@Christian is this what you mean?
There are 6 disks total on my container.
Could I… delete the mystery volume?
I had occasion to reinstall Catalina many times earlier this year while trying to diagnose a hardware problem. This happened to me every single time.
Really a great user experience, eh?
Yes. That looks weird.
If you have a backup and after having read what @jec0047 has posted: Yes, I would try deleting it like it is explained in the discussion @jec0047 has linked to. Just be sure to have a backup to be safe, if it turns out not being a good idea… Just in case…
Don’t worry about it and certainly don’t try to delete anything. It all looks like it should (at least it’s just like our three Catalina systems, and they are working fine). Remember – if it ain’t broke, don’t fix it.
macOS Catalina automatically creates a system partition. So when you re-install it without deleting that partition, you get another new partition. It’s such a headache.
The only way to get rid of it is by deleting the partition during the installation phase. But that will delete all the data so make sure to make a fresh back-up on an external disk before you do that.
After doing a reinstall on one system and seeing the duplicate system partitions (and the partitions wrongly named, claiming two data partitions with one being the system partition), I verified that the original system partition was not being used. Then I simply deleted it and renamed the good partitions back to Macintosh HD and Macintosh HD - Data.
Alrighty. Thanks for the help guys.
Regarding the “don’t delete anything”: well, it’s taking up disk space, right? And that’s in short supply for me.
Does Time Machine = good backup?
A few months ago after a Catalina reinstall I had 2 mystery disks. I did a Time Machine backup and made a clone with Carbon Copy Cleaner. Only then did I try to delete the mystery volumes. It worked fine for me. I would only try this after making sure I had a good clone. If something should go awry, a clone would get you back up and running quicker.
I would vote for a clone, too.
Time Machine is nice for versioning, but I do not trust it as far as being a “real” backup. Then again, it is Apple’s way of doing backups, so you should be fine, I guess.
You really need both. TM is a good first line of defense and it’s easy to recover the odd lost/accidentally deleted file, but to rely exclusively on TM is not wise. Nor is it wise to rely on a single clone backup.
