I don't know the term P2...
Partition 2?
SATA port 2?
Sounds BIOS-specific.
(EDITED to correct followinghistory — beware I've cobbled this story together from bits and pieces so it prolly has errors)
So WRT Catalina, there are some tricky angles. Apple did two big things with that version:
- Introduced APFS as default and required drive format. APFS was introduced around Sierra but you could avoid up through High Sierra.
And...
- Introduced read-only System Volume. CCC could still work with these but it was getting more complex due to APFS container volume of system and "Data" partitions, firm links between user apps/data and the system, APFS snapshots, and APFS TimeMachine implications.
Firm-linking is a way of connecting immutable system files between the SSV to user modifiable files in Data while providing a folder structure that looks like pre-SSV versions of macOS. Basically folders need to work while including and hiding the detail that some files can't be changed and there's no mechanism that can possible override them.
This all made thinking about user data backup much trickier. CCC could still do an incremental bootable clone, but CCC incremental clones became unreliable because macOS updates could cause the backup drive's system and data partitions to get out of sync, depending on how the user did backups. At that time Bimbich began to face the implications of APFS by recommending a Data-only backup and recovery with OS reinstall and Migration Assistant.
Then in Big Sur read-only system volume became Signed System Volume (SSV) where Apple crypto signs the read-only system. CCC incrementals cannot work with this because CCC can't manage the signing. Apple APFS Replicator can, but it's not designed for incremental backups. From there CCC incremental bootable backups were no longer possible and Bombich added Legacy Bootable Backup Assistant, which employs Apple APFS Replicator with its harsh constraint of a target reformat, full copy no incrementals no exclusions, and I/O errors cannot be tolerated.
SSV is above and beyond other features like read-only system, SIP and Gatekeeper: it locks the core system files from being modified by anyone but Apple. How crypto is applied is beyond my understanding, and I am offering an oversimplified view.
So what we face as users is that when you say "Catalina" you could be talking about classic macOS or SSV macOS.
From perspective of understanding UEFI booting this change is subdued, because most of the differences appear after macOS has booted. But from perspective of drive structure and tools for drive mgmt it matters a lot! For example, HFS+ boot and APFS boot layout work very differently and filesystems are covered by different OC modules. Vintage Macs required a firmware update from Apple to handle APFS even before SSV was introduced.
So any troubleshooting can get confusing and this Catalina evolution is especially so.
At Big Sur it is APFS SSV only. But Apple didn't fix things like installation drive renaming until Monterey. So there are big and little pitfalls.
You are talking about how BIOS presents drives, which I find confusing because once you choose an OC partition as visible in BIOS, BIOS naming no longer matters as the drives are reckoned by OC or whatever bootloader.
So it's important to get terminology clear.
