Valve’s rule for a microSD card in the Steam Deck fits in one sentence from its support guide: “Any UHS-I (class 3 or better) microSD card is supported.” Format the card on the Deck itself, and it is ready for games. The hard part is reading the label, because a microSD card carries several marks and only one of them answers Valve’s “class 3”.
We have not compared cards in a Deck for this guide, and we list no brands or prices. The rules come from Valve’s support pages and tech specs; the meaning of each symbol comes from the SD Association, the body that writes the SD standards. All of it was checked on October 16, 2026.
What Valve supports
Valve’s support guide says its information applies to both the Steam Deck LCD and the Steam Deck OLED unless it notes otherwise, and the microSD section notes no difference.
| Item | What Valve says |
|---|---|
| Slot | High-speed microSD slot on both current storage configurations |
| Bus | UHS-I |
| Card types | SD, SDHC and SDXC |
| Speed | Class 3 or better |
| UHS-II and UHS-III cards | Expected to work, but their higher transfer speeds are not supported |
| File system | ext4 with casefolding; the Deck formats the card itself |
| Booting | The Deck can boot an operating system from microSD |
What the symbols on a card mean
A microSD label carries several marks, and each one answers a different question. The SD Association defines all of them.
Capacity: SDHC, SDXC, SDUC
The name on the card tells you its size range. SDHC covers cards over 2GB up to 32GB. SDXC covers cards over 32GB up to 2TB. SDUC starts above 2TB. Valve lists SD, SDHC and SDXC, so it does not list SDUC.
Bus: UHS-I and up
The bus is how fast data can move between the card and the device. The SD Association lists UHS-I with a top bus speed of 104MB/s, in its SDR104 mode. UHS-II and UHS-III cards add a second row of pins for faster transfers. The Deck uses UHS-I, which is why Valve says those faster cards work but do not run at their higher speeds.
Speed class: “class 3 or better”
Speed class marks give a minimum sequential write speed. They were made for video recording, where a card must keep up with a constant stream. The SD Association defines three families:
| Minimum write | Speed Class | UHS Speed Class | Video Speed Class |
|---|---|---|---|
| 2MB/s | C2 | – | – |
| 4MB/s | C4 | – | – |
| 6MB/s | C6 | – | V6 |
| 10MB/s | C10 | U1 | V10 |
| 30MB/s | – | U3 | V30 |
| 60MB/s | – | – | V60 |
| 90MB/s | – | – | V90 |
Valve does not spell out which “class 3” it means. The only class in the SD Association’s tables with a 3 is UHS Speed Class 3, drawn as a 3 inside a U. That mark, U3, means at least 30MB/s sequential write. V30 is set at the same 30MB/s.
App performance: A1 and A2
The A marks are about small random reads and writes, the kind of access an installed app makes. The SD Association sets these minimums:
| Class | Random read | Random write | Sustained sequential write |
|---|---|---|---|
| A1 | 1,500 IOPS | 500 IOPS | 10MB/s |
| A2 | 4,000 IOPS | 2,000 IOPS | 10MB/s |
IOPS here means 4KB read or write commands per second. A2 reaches its figures with command queuing and a cache, and the SD Association says A2 performance needs both an A2 card and a host that supports A2. Valve’s pages say nothing about A1 or A2, so treat an A mark as a bonus, not a requirement.
How to format a microSD card on Steam Deck
A new card has to be formatted before the Deck can use it. Formatting erases the card, so copy anything you want to keep first.
- Insert the card into the slot on the bottom of the Deck.
- Press Steam and open Settings > System.
- Select Format SD card.
- Wait. Valve says the process may take a few minutes.
- Open Steam > Settings > Storage to see the card next to the internal drive and manage what is on each.
The Deck formats the card as ext4 with casefolding, a Linux file system. You do not need to prepare it on another computer: Valve’s FAQ says the Deck formats SD cards to the proper format.
What to look for on the label
- microSDXC if you want more than 32GB. Valve does not list SDUC.
- UHS-I or a newer bus. Newer buses work but run at UHS-I speed in the Deck.
- U3, the mark that matches Valve’s “class 3 or better”. A card with only U1 or C10 promises 10MB/s, below that line.
- A1 or A2 if you want the random-access minimums above. Valve does not require either.
Before you open the Deck
If you plan any hardware work, take the card out first. iFixit’s Steam Deck guides warn that a card left in the slot while you remove the back cover could snap in half. Our guide to Steam Deck mods covers that work and Valve’s warranty warning.