SteamOS/Windows Dual Boot on SSD

I wrote this comment for somebody else with a similar question so I'll copy and paste, but I unfortunately am also a filthy Genshin player and the TLDR is it'll work fine. One thing to note is I would recommend installing to the SSD if possible because installs on SD cards will significantly more quickly kill the SD card due to how much reading and writing Windows does. Copy and paste below, but feel free to ask questions!

I replaced an aging laptop with mine, and I have to say, Windows is fine on the Steam Deck. It's obviously not going to have all the same bells and whistles but performance for most games is the same and it is more flexible on compatability since it runs natively instead of via Proton.

I've been using dual boot for this and started since I got my Deck in early July last year, and a lot of the hate is really overblown. It's good if you just want to use the Deck the same way as any other computer or with applications that cannot work in Steam OS and it's a completely fine experience, and applications like Handheld Companion or Steam Deck Tools make it simpler.

I would just say that it's up to preference and need since it does need setup and tweaking, so if all you care about is playing a few games that work fine in Steam OS, there is not much reason to dual boot for Windows and Steam OS is simpler to set up. But if you want more freedom on application choices and compatability or even just to use it for more everyday purposes like any other computer, Windows runs perfectly fine on it. It will be ultimately up to preference and use case.

As someone that plays Genshin, it actually runs fine through proton funnily enough. Honkai Star Rail on the other hand…
- Main issue with that is that it requires bypassing the anti cheat which can get people banned. There aren't many reports of this so most say it's not really common, but bans do happen from Hoyo for doing so on the Steam Deck; there was a post on the Steam Deck reddit a while back with somebody who got banned on Star Rail for it even though they had got it running as well as Genshin. Was pretty rough, they quit all Hoyo games because of it even though they were a whale.
  
  I wouldn't risk it myself for that reason, so I've always played it on Windows. Though, since I use my Deck as my main computer, I'm most often in Windows since so much software I need either barely works or is broken on SteamOS.

Dual booting from the same drive is usually pain, from my experience.

For the deck, you could boot Windows Portable off a micro sd card

Why do you want Windows? Specific game? Most can be installed without Windows.

Genshin Impact. All methods to get it to run on SteamOS/Linux are too risky for my account IMO
- I see! Not possible to add the installer as a Steam Game and afterwards add the launcher? Just like the guides for Battle.net client? That's a bummer then!

Windows doesn't always play nice, windows updates will frequently break the bootloader and prevent SteamOS from booting. If you don't plan to use windows as your main OS I'd probably recommend installing it to a microSD. Performance may take a bit of a hit, but it's safer and much less likely to cause issues with the device as a whole.

I wouldn't say it's safer since it will significantly more quickly kill the SD card itself, but from a year of using Windows and SteamOS, the bootloader break can be solved pretty easily since there's an easy script on SteamOS to fix it and you can always disable Windows updates if they are annoying. Ultimately a matter of preference on what's preferred though, but just good to know the pros and cons of each option!
- Nicer microSD cards now claim to have comparable or better numbers of write cycles compared with average SSDs. Samsung claims their nicer cards have 100,000 writes per sector for example, while many SSDs seem to report having 40,000-100,000.
  
  Unless I'm misunderstanding something it seems like running windows on a microSD should be fine. You can always go with a cheaper card too if you want low risk.

I set up a Windows install on one of these:

https://www.kingston.com/en/ssd/xs2000-portable-usb-c-solid-state-drive

Works great, no complaints. Boot from Steam Deck for Steam Deck things, boot from external SSD for Windows things.

I COULD have done a dual boot from the internal SSD, but I wanted to keep it clean.

I was dual booting Windows / Linux on the same drive on my laptop for a while (before I got a separate drive for each of them), Steam Deck should be no different.

Good news is that it pretty much works flawlesly. Bad news is that Windows really liked to mess up Grub after almost each update. It requiring to manually reinstall / reconfigure it.

That was pretty much it from what I remember.