Problem
Recently, my motherboard died. I had a backup, but it would be easier to recover if I could imply take my NVMe SSD and my HDD to another computer and boot it. I did exactly that with an MSI, no problems. I had Manjaro installed since 2020.
Then I tried using a 2017 Acer Aspire. I fiddled with secure boot, I reinstalled and updated grub using a live USB, I even tried manually editing EFI entries with efibootmgr. The motherboeard would only detect the Windows Boot Manager.
I took the nuclear approach of deleting all Windows partitions, leaving only my /boot and / partitions. It somehow kept trying to boot into Windows.
I officially quit the task and decided to try more cumbersome methods.
Solution
Turns out this was a bug on the firmware, probably.
I was going to use clonezilla to copy partitions to an external drive, but the I realized it has an option to boot to an existing OS. It was finally able to boot.
I come here just to register this workaround, as I had not seen this out in the Internet. I could keep a USB stick with clonezilla always plugged in just to boot. I believe the real “solution” would be a clean reinstall, as I know the hardware is fully supported on Linux and at one time it had a Mint version.