Edit. My bad, didn’t realise PARTTYPE was showing.
You have four duplicate UUIDs for your root volume. Everything else also has a duplicate. It’s like partitions were randomly copied all over the place.
I have no idea how you were booting before. Even if you remove the HDD, you got two same UUIDs for your ext4 root partition.
~~
I removed the Linux partitions on the HDD and will now reinstall Manjaro on the SSD.
/dev/sdb1 gpt de94bba4-06d1-4d40-a16a-bfd50179d6ac ntfs Windows recovery environment
and
/dev/sdb5 gpt de94bba4-06d1-4d40-a16a-bfd50179d6ac ntfs Windows recovery environment
/dev/sdb2 gpt c12a7328-f81f-11d2-ba4b-00a0c93ec93b vfat EFI System
and
/dev/sdb7 gpt c12a7328-f81f-11d2-ba4b-00a0c93ec93b vfat EFI System
/dev/sdb4 gpt ebd0a0a2-b9e5-4433-87c0-68b6b72699c7 ntfs Microsoft basic data
and
/dev/sdb6 gpt ebd0a0a2-b9e5-4433-87c0-68b6b72699c7 ntfs Microsoft basic data
these are also identical (duplicates)
I didn’t notice that, interesting!
They do have different sizes however. I don’t think I need to mess with that and I will let Windows be Windows on the HDD.
I am using Linux 99.9 % of the time and I will be happy with a clean Manjaro installation.
The Linux kernel cannot tell apart partitions with identical UUIDs.
I’m pretty sure the same applies to Windows.
The only thing you can expect from this is pure random behavior and chaos.
assign new UUIDs so that each one is unique
I used blkid to get the UUIDs of the partitions and figured out that the third column of the lsblk output is not the UUID of the partition but of the partition type. All partitions of a certain type have the same partition type UUID. E.g. every EFI system partition has the partition type UUID c12a7328-f81f-11d2-ba4b-00a0c93ec93b (see here for a table showing the UUIDs of different partition types).
But your posted output stated the command
which, of course, does not even list the UUID
lsblk -f
for instance, would show that
and
blkid
without parameters would also look different
nice confusion 
They’re PARTTYPE UUIDs…they’re the same because they’re the same type.
My system is up and running again. I was even able to keep my home partition so it immediately felt like home again.
Even though no graphics driver seemed to be installed (at least that’s what mhwd -li said) at first. I then ran mhwd -a pci free 0300 to install free drivers and everything seems to work just fine.
I present to you, a much cleaner system than before:
david-ssd% mhwd -li
> Installed PCI configs:
--------------------------------------------------------------------------------
NAME VERSION FREEDRIVER TYPE
--------------------------------------------------------------------------------
video-linux 2024.05.06 true PCI
Warning: No installed USB configs!
david-ssd% lsblk -o PATH,PTTYPE,PARTTYPE,FSTYPE,PARTTYPENAME
PATH PTTYPE PARTTYPE FSTYPE PARTTYPENAME
/dev/sda gpt
/dev/sda1 gpt 0fc63daf-8483-4772-8e79-3d69d8477de4 ext4 Linux filesystem
/dev/sda2 gpt 0657fd6d-a4ab-43c4-84e5-0933c84b4f4f swap Linux swap
/dev/sda3 gpt 0fc63daf-8483-4772-8e79-3d69d8477de4 ext4 Linux filesystem
/dev/sda4 gpt c12a7328-f81f-11d2-ba4b-00a0c93ec93b vfat EFI System
/dev/sdb gpt
/dev/sdb1 gpt de94bba4-06d1-4d40-a16a-bfd50179d6ac ntfs Windows recovery environment
/dev/sdb2 gpt c12a7328-f81f-11d2-ba4b-00a0c93ec93b vfat EFI System
/dev/sdb3 gpt e3c9e316-0b5c-4db8-817d-f92df00215ae Microsoft reserved
/dev/sdb4 gpt ebd0a0a2-b9e5-4433-87c0-68b6b72699c7 ntfs Microsoft basic data
/dev/sdb5 gpt de94bba4-06d1-4d40-a16a-bfd50179d6ac ntfs Windows recovery environment
/dev/sdb6 gpt c12a7328-f81f-11d2-ba4b-00a0c93ec93b vfat EFI System
/dev/sdb7 gpt ebd0a0a2-b9e5-4433-87c0-68b6b72699c7 ntfs Microsoft basic data
It feels awesome to have a working GUI again.
Thank you all for your help!
I’m very happy for you!