He has a dual boot setup running using rEFInd, and while that survived the OS update just fine, when he wanted to return to SteamOS after a quick stint in Windows today, he was greeted by a GRUB boot menu.

Detective foosel to the rescue.

Attempting to boot the SteamOS entry in grub resulted in an error like this (with another device UUID):

error: no such device: aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee
error: file `/boot/vmlinuz-linux-neptune` not found.
error: you need to load the kernel first.

Press any key to continue...

So it couldn’t find it’s boot device and due to that also not the kernel stored thereon.

Entering the Deck’s boot manager and manually booting \efi\steamos\steamoscl.efi also led to the same situation.

I was able to still boot into SteamOS via the fallback entry however (which also had a different boot device UUID).

And it took me way too long¹ to simply just get the idea to let Linux update its GRUB entries:

sudo update-grub

That fixed it.

No idea if the dual boot setup played a roll in this mess or if it was just some random hiccup, my Deck’s update went without a hitch 🤷‍♀️ But if it happens again I now have this entry to check 😁