Practically every tutorial on the internet for dual booting Steam OS and Windows on Deck tells you to use the Steam Deck recovery image to shrink the home partition so you can make a new partition for Windows. I did this first try on my previous Steam Deck, but I had to send that one back to Valve because of hardware problems. On this one, Steam OS just dies every time I shrink the home partition. Trying to boot it gets me stuck on an endless Steam Deck logo screen. It’s so bad that re-installing Steam OS doesn’t even work and I have to reimage the system. Is there something that I’m likely doing wrong?
If you’re reflashing the whole drive anyway, partition it before you start.
I found the problem. It turns out that my mistake was trusting KDE Partition Manager. GParted Live worked first time.