Resolved Mojave/Catalina Mystery Partition?

[TheShortTimer]

My MBA is currently set up with Snow Leopard, Mojave and Catalina (which had been upgraded from El Capitan.) When booting into Mojave and Catalina not too long ago, I spotted that the list of partitions has changed. There is now an additional 106GB partition called "El Capitan - Data" and in Mojave it's mounted on the desktop.

This is really confusing because I do not understand why or how this partition exists because the SSD is only 256GB and as you can see, 40GB is occupied by Snow Leopard and Mojave and Catalina have 106GB and 107GB respectively so where would the space for 106GB for this additional partition have appeared from?

Also, the mystery "El Capitan - Data" partition is not mounted on the desktop in Catalina but it is listed in Disk Utility:

Does anyone know what's going on here?

 

[Wowfunhappy]

🤔

Catalina and above split the root boot volume into two separate partitions, which are then made to look like one in Finder. I assume this has something to do with that. But I have no clue where the "El Capitan" is coming from.

I assume you had an El Capitan partition at some point? (Not that that really explains anything.)

...can you delete it? Uh, after imaging the disk, in case it breaks something.

 

[Needleroozer]

It looks like a couple things are happening.
1. macOS after 10.13 can install to APFS-formatted drives, which allow multiple volumes to share the same container. This means that one 106GB container can have multiple volumes (“Catalina” and “El Capitan - Data”) coexisting in it. Either can expand to fill the container, but you don’t have to worry about resizing these volumes like you did with partitions. It looks like your Catalina APFS container actually has five volumes contained within it, from your screenshots (I think this is normal behavior for Catalina).
2. macOS Catalina and later have divided the macOS install into a read-only System partition and a normal Data partition, rather like an iPhone. This makes it much harder to modify the System partition directly, theoretically allowing for greater security and reducing the chance of accidents.
As you can see in your last picture, your user data for Catalina (10.15.7) is being stored on the “El Capitan - Data” volume (note the home icon). It’s likely named thus because your El Capitan install was on a partition named “El Capitan” and so when it was updated and replaced by an APFS container, the installer just appended “ - Data” to the partition’s old name.

So older versions of macOS will mount this new Data volume as just another volume, but Catalina and later auto-mount it internally as the home of your user data.

 

[Needleroozer]

🤔

Catalina and above split the root boot volume into two separate partitions, which are then made to look like one in Finder. I assume this has something to do with that. But I have no clue where the "El Capitan" is coming from.

I assume you had an El Capitan partition at some point? (Not that that really explains anything.)

...can you delete it? Uh, after imaging the disk, in case it breaks something.

Yikes - guess I spent too long typing...

I definitely recommend not deleting the “El Capitan - Data” volume, since it definitely looks like your Catalina install’s data volume and has all your user-specific files and applications.

 

[K two]

🤔

Catalina and above split the root boot volume into two separate partitions, which are then made to look like one in Finder. I assume this has something to do with that. But I have no clue where the "El Capitan" is coming from.

I assume you had an El Capitan partition at some point? (Not that that really explains anything.)

...can you delete it? Uh, after imaging the disk, in case it breaks something.

That's a misnamed Catalina - Data. You can correct the name in Disk Utility. Your mystery is normal.

 

[TheShortTimer]

Thanks for so many swift replies!

@Wowfunhappy yes, there was originally an El Capitan partition and Catalina performed an upgrade installation on it.

@Needleroozer ok, I definitely will not be deleting the partition - the thought had crossed my mind but fortunately I didn't act on it! Thanks for the overview, I understand the background now.

@K two I'll do that but I'm curious as to why Mojave suddenly began mounting it.

Thanks again for the help.

 

[Wowfunhappy]

Yikes - guess I spent too long typing...

I definitely recommend not deleting the “El Capitan - Data” volume, since it definitely looks like your Catalina install’s data volume and has all your user-specific files and applications.

Thanks—note, I did recommend imaging the disk first for a reason!

 

[TheMacNovice32]

Hello, I have a similar problem. I just recently noticed a random 250gb T-Mojave5 partition and when booted into it is locked with a password any suggestions on what this could be? there are 14gb taken up on this partition.

thank you