I'd personally say switch the OS X side to the FW 800 drive and then use the whole internal drive for Windows, or at the most leave a vestigial 10 GB partition for OS X if an install of it has to be ...
One of the advantages OS X has over Windows is the easy ability to install OS X on practically any supported local volume, meaning that regardless of the hardware, if the volume can be mounted locally ...
It's worth noting that Windows is not perfect on the Steam Deck. However, installing the OS or booting it from a microSD Card, can turn it into a nice portable Windows PC. Through this article, you ...