Steam Deck

The Steam Deck is spec'ed with a UHS-1/SDHC slot, which means that you can't use SDUC-class cards and you won't get much benefit from using comparable cards with a UHS-II/UHS-III bus mark compared to one with a UHS-I mark, even if the other marks otherwise suggest better performance. You can basically ignore the A/V markings because they're not granular enough to help with comparing cards at this particular performance level (you should instead compare "Random Read"/"Random Write" performance benchmark scores).

Note that there remains a considerable amount of variance among similarly marked cards. For example, the Sandisk Extreme Pro (Bus: UHS-I, Speed: 3) can benchmark write speeds which are almost twice as fast as the Sandisk Extreme (Bus: UHS-I, Speed: 3).

tl;dr: The ideal card will have the following markings:

• Capacity Standard: SDXC (SDUC is not compatible)
• UHS Bus Speed: I (higher is fine, but not helpful)
• Speed Class: 3 (though you should really be comparing benchmark scores instead!)

I can't say the class or anything like that, but... A lot of the real performance depends on factors that are well outside of any general consumer specified marketing. There used to be a YT conference video by Andrew Bunny Haung about the security of SD cards I would link to. It was only unlisted for years, but has been set to private now. SD cards run an entire internal microcontroller, usually an 8051 clone. These can be hacked, and he showed this. The issue will never get addressed as a security bug either. The microcontroller onboard is managing the actual page blocks in flash memory. A lot of low cost and low spec/smaller storage sizes are the same dies used for top specification units, but have a range of problems that do not fit within a simplified marketing spec. Like if part of the storage area on the die has issues with write speed, depending in the severity of the issue, the slow block may be masked off as if it does not exist, or the entire card may just get sorted and sold at a lower speed rating. This is why bench marks are junk data.

Your best bet is to buy cards from brands that actually own their own memory chip fab. If you also buy their high end offerings and largest capacity, (I know, sounds like I work for these guys as a shill, but I don't), you are much more likely to get their absolute best performance possible. These will contain the best dies from the fab yield and will not have the types of problems that may exist in smaller sizes and lower classes. They usually do not make several dies specifically for all the various sizes and classes you see for sale. There are only a few dies made, and the defects determine the end rated product.

Samsung Evo 512 is really good for the Steam Deck. Fast & reliable.

I bought an "U3 V30 A2" SD card and it works great. I might be wrong, but I think I've heard that it's useless to go anything faster.

Also, the most important thing: buy from a reputable source. There are A LOT of fake SD cards out there.

I remember reading a reddit post that led me to picking this card. I've been using it for a while now. No complaints at all.

https://www.amazon.com/dp/B0BDZ4GRB5