Nandroid backup - System and vendor won't mount

[fuzzychicken]

Hello folks,

Now that Pixel XL is not going to get any updates from google, I am ready to switch over to the ROMs. I thought I should take a nandroid backup before proceeding. This device has unlocked bootloader and rooted with Magisk. Its running September update. I didn't install the October update.

Booted Pixel XL into TWRP using fastboot. Used twrp-3.3.1-3-marlin.img. When I try to backup, I get an error
Failed to mount '/system' (invalid argument)
Failed to mount '/vendor' (invalid argument)
It manages to generate the digest but again gives an error, failed to mount system. Backup failed.

I even restored the phone to factory settings and I get the same error.

Is there anything else I should be checking?

Edit: Tried older TWRP, 3.3.1.0. Now I get an error with the same two, device or resource is busy.

Thanks in advance.

 

[NZedPred]

You probably only need to backup your data (which may require you to remove your screen lock before boring into TWRP). Optionally backup the boot partition so you have any custom kernel and Manish backed up as well.

The system, vendor, and other partitions can be restored by fastboot. Less than ideal, but that's what I've been doing.

 

[fuzzychicken]

You probably only need to backup your data (which may require you to remove your screen lock before boring into TWRP). Optionally backup the boot partition so you have any custom kernel and Manish backed up as well.

The system, vendor, and other partitions can be restored by fastboot. Less than ideal, but that's what I've been doing.

Thanks. So you started having this problem with Android 10 only or it was before android 10 as well?
The second question I have, if it won't mount system, is that partition the same as the data, because android has Slot A and B. This is the first phone I am dealing with that has 2 slots. Any idea if this create any issues at the time of ROM / Lineage OS install?

 

[NZedPred]

Thanks. So you started having this problem with Android 10 only or it was before android 10 as well?
The second question I have, if it won't mount system, is that partition the same as the data, because android has Slot A and B. This is the first phone I am dealing with that has 2 slots. Any idea if this create any issues at the time of ROM / Lineage OS install?

Myself I haven't tried mounting either as writable, but read only for system is fine. I've never flashed a custom rom. I do see posts from other people with the issue though.

There is only one data partition, and it is completely separate from system etc, and slots don't affect it.

Just follow the instructions for the custom rom you end up installing.