Should be a caution notice at the manual that SSD shouldn't have SteamOS traces not only at installation to microsd, but when its updating too

[Metafalica]

I had SteamOS installed on microsd from this guide for a long time.
I had Windows installed on SSD.
Then I decided to put back old SSD with SteamOS for a while.
Then I did boot from microsd.
SteamOS found updates, I confirmed them, and they got installed.
After reboot I couldn't boot SteamOS (it was total mess about missing almost everything at boot) - as I understand, it tried to boot from SSD.
I did force shutdown Deck and turn it ON via choosing boot device (microsd), but it still didn't boot with same errors.
So I replaced SSD again and reinstalled SteamOS to microsd. This time it updated to the latest successfully.

Just saying, that SSD shouldn't have any traces of SteamOS at all or else you will corrupt SteamOS installed on microsd when it updated.