Not enough disk space to install - how to Remove extra volume?

My install routine say “not enough disk space to install”
I’ve discovered that there is an extra volume* on the boot drive that almost fills the drive.
Two volumes display in Disk Utility (but other utilities show 5 volumes)
One is current install - Catalina
One volume is my old Mojave install. 95GB

Is there a way to remove the old volume without reformatting the entire disk?
I don’t want to loose all my installed applications.

*New information: there are other examples on apple developer forums - some Installations of Catalina have created a copy of the programs folder in a second DATA volume. It doesn't show up in the normal finder window.

I booted from my ccc backup.
used Disk Utility to remove the Mojave volume.
Tried to reboot from the installed Catalina - FAILED - I broke something.
- perhaps Clover still had some files on that old Mojave volume. (nope! - that is just the Data partition)
my Big Sur install usb was ready, so I tried to install.
after the Mac install window, clicked accept. selected install drive, It started install then
- it produced an Access Denied message - would not install Big Sur. (damaged Data partition?)

Now trying to restore Catalina from Time Machine.
(of course that will also restore all the Mojave junk too - 95GB, that will take time)

any suggestions?
===
successfully booted up from restored Catalina.
now that I look at it again, perhaps the Data partition has retained the partition name from the Mojave install.
- and the disk is just full of stuff. (Original build was 10 years ago. 130GB drive. I think folks use a larger SSD now for main OS install.
but that doesn't explain why there are 5 different volumes.
- will installing Big Sur with OpenCore fix any goofy partitions?

I think I have now removed enough junk from the install disk to allow the install of Big Sur.
- should I try it again? - tried. didn't work. Need 5GB more space.
- waiting for new 500gb SSD

if I do a Clean Install with a new SSD...
Will applications work if I do a clean install of Big Sur on my new drive, and then restore the applications folder from the CCC backup?
- I don't think you can copy an application from one mac and install it on another mac. it has to be "installed".
BUT, will applications work if I RESTORE them from my ccc backup?

More Information on disk space:
The second volume is indeed the - Data volume. (normal in Catalina)
but there is something wrong.

the System/Volumes folder contains a second OSX Catalina disk Volume. (see images) with another system folder.
it's (almost) a complete copy of the system folder. it contains a fully populated Applications folder.
- I can launch apps from this folder!
- #2 Applications folder is 54GB

Also: #2 OSX Catalina volume also has a complete Library folder (duplicate of #1 OSX Library folder)

so the question is still - how to fix this.
- and/or How do I remove this second volume (without breaking the OS)?
(a clean install on a new drive will fix it, but then I may have to re-install lots of applications)

As I have no idea what´s up with your boot drive I would suggest a different approach, as you already (and reasonably) decided to make a fresh install on a new SSD.
First install a clean Big Sur on your new SSD. Thereafter I would make a (bootable, which seems to be tricky with Big Sur though) copy of that clean system onto a secondary HD with CCC. Just as a quick fallback if something goes wrong in the next step with your garbled old system.
Now, booted into Big Sur from your new system SSD, use the macOS migration assistant to handle the transfer of your system and user account data. This hopefully allows you to get rid of the doubled data with the help of the selection menu within it. Most copied applications should be running without issues, some might have to be reactivated (but not reinstalled), some might require an update (maybe a good idea to be on the last available version(s) for your catalina system prior update).
If you are not happy with the result of the migration, just replace your Big Sur install with your clean backup from HD and try again.

Say Heading said:

As I have no idea what´s up with your boot drive I would suggest a different approach, as you already (and reasonably) decided to make a fresh install on a new SSD.

Click to expand...

I was not aware of Migration Assistant. Thank you!
(there it is in my Utilities folder)
That makes it much more comfortable to make a clean install.

Thinking about the goofy extra volume, I suspect it was caused by multiple (failed) attempts to install Catalina and my poor understanding of config.plist combined with a poor understanding of Clover.
Getting ready to install Big Sur, I was really intimidated by the OpenCore instructions and my limited understanding of Terminal commands, but as others have said, yes, OpenCore is the way to go. It allows (helps) you to set up all the necessary parameters for a new install. Pay very close attention to the instructions. And learn about Terminal.

Stuck on ASUS BIOS screen!
I successfully installed Big Sur on my SSD.
made a Time Machine backup of the new install.
Used Migration Assistant to copy Programs folder from CCC backup to new Big Sur install.
Migration Assist ended (I think) and restarted.
Now system is stuck at BIOS screen - I cannot enter BIOS with F2 or DEL key.

Solution?
currently looking into the USB BIOS Flashback button on motherboard. - (it's in the ASUS manual)

No way to enter BIOS? I´m not really having an idea, but what I would do in that case:
Remove all (ALL! - including hidden dongles, old logitech unifying transmitter, everything) unnecessary USB peripherals, just use an USB cable-keyboard, try different USB ports (USB flashback port preferred, see manual), maybe also unplug SATA devices if the first steps don´t help, just a bare motherboard. CMOS battery? Just in case.

Caution for using Migration Assistant
My BIOS was somehow corrupted. I'm pretty sure it was the Migration Assist process - when it ended, I think the program was trying to do a special ReStart (the mac way) - which changed the BIOS.
After I reset the bios and rebooted, the Migration Assist posted a message "Successful..." .

I used the USB BIOS Flashback program to reset the Bios - used a current bios downloaded from ASUS site.
- Note: the processes requires you to download the BIOS Firmware AND a special renaming program. The current download did not contain the rename program. I had to download an older version of the BIOS which included the rename program. Flashback will not work without renaming the bios file. (ya, I tried - it failed)
*note, also that you must run the rename executable on a PC. (put both files in the same folder, then run the rename program)

I recovered my mac program files, but they all need to have the validation code input before they will work. - that's a hassle finding them because my email didn't transfer. I have to set it up from scratch.

michaelbp said:

Caution for using Migration Assistant
My BIOS was somehow corrupted. I'm pretty sure it was the Migration Assist process - when it ended, I think the program was trying to do a special ReStart (the mac way) - which changed the BIOS.
After I reset the bios and rebooted, the Migration Assist posted a message "Successful..." .

I used the USB BIOS Flashback program to reset the Bios - used a current bios downloaded from ASUS site.
- Note: the processes requires you to download the BIOS Firmware AND a special renaming program. The current download did not contain the rename program. I had to download an older version of the BIOS which included the rename program. Flashback will not work without renaming the bios file. (ya, I tried - it failed)

I recovered my program files, but they all need to have the validation code input before they will work. - that's a hassle finding them because my email didn't transfer. I have to set it up from scratch.

Click to expand...

Probably not Migration Assistant, I use it often and it works perfectly. Sometimes OpenCore likes to mess with the bios, or at least I've had some issues on a couple Asus boards but newer than yours.