The card reader sets the ceiling, not the box
Valve's guidance for the Steam Deck is that any UHS-I class 3 or better microSD card is supported. UHS-II and UHS-III cards may work, but Steam Deck does not support their higher transfer speeds. A pricier card therefore buys you capacity on a Deck. It does not buy you speed.
Several independent guides put the practical slot ceiling at roughly 100 MB/s. One example is a UHS-I bus limit of approximately 100MB/s. Treat that figure as community and third-party guidance. It is not a Valve specification. The SD Association lists UHS-I at 104 MB/s as an interface-mode figure, not a guaranteed real-world result.
ASUS's handhelds are different. ASUS says the ROG Ally, ROG Ally X, ROG XBOX Ally, and ROG XBOX Ally X all feature a UHS-II microSD card slot. The ROG XBOX Ally X20 instead has a microSD Express slot. ASUS also warns that UHS-II cards inserted into the X20 will run at slower UHS-I speeds, because UHS-II and microSD Express are separate standards.
| Device | Card reader | Internal drive (per ASUS) or note |
|---|---|---|
| Steam Deck OLED | UHS-I (Valve) | 512 GB or 1 TB SSD |
| ROG Xbox Ally (2025) | UHS-II | 512 GB M.2 2280, upgradeable |
| ROG Xbox Ally X (2025) | UHS-II | 1 TB M.2 2280, upgradeable |
| ROG Xbox Ally X20 (2026) | microSD Express | 1 TB M.2 2280, upgradeable |
The Deck and ASUS rows come from ASUS's own table and Valve's specifications. The Steam Deck drive swap is a different story, and I cover it below.
Format the card in the device that will use it
The two ecosystems want different filesystems.
- Steam Deck: the Deck formats cards as ext4 with casefolding. In Steam, go to Settings, then Storage, choose the card and format it there.
- ROG Xbox Ally and Ally X: ASUS support says the card must be NTFS. Formatting erases everything, so back up first.
ASUS's steps in the Xbox full-screen experience:
- Insert the card in the slot near the headphone jack.
- Tap the AutoPlay pop-up and choose the card.
- Select Open folder to view files.
- Press and hold the card in the sidebar and choose Format.
- Pick NTFS in the file system dropdown and choose Start.
If you missed the pop-up, you can format from Windows desktop mode in File Explorer. After formatting, create a new folder for your games.
ASUS's FAQ is scoped to the RC73 models, which are the 2025 Ally and Ally X. Its newer SD-card guide also says to format as NTFS and covers the X20. The practical rule is to pick one filesystem per card and keep the card with its device. An ext4 card from a Deck will not be readable on a Windows handheld without extra drivers.
Card or internal SSD?
ASUS's own advice is that upgrading the internal SSD gives you the most seamless experience. A card is the no-screwdriver option, so ASUS suggests trying a card you already own first. That is a manufacturer's view, but it matches basic storage logic. One drive means one speed profile, and cards are weaker on the small random reads that games issue.
Cards do make sense for large, rarely patched titles on a device with a fast reader. Keep shader-heavy games on the internal drive.
If you're on the Deck, A2-rated UHS-I cards are the sensible target. One guide notes that an A2 rating is preferred because it improves random read and write performance.
Before you open a Deck
Valve says it does not recommend opening or modifying the Deck, and that damage from such work is not covered under warranty. Valve's guide lists the screwdrivers: Phillips size 1 for the LCD model and T6 Torx for the OLED. This is not the same as saying every warranty is automatically void, so read the terms for your region. The available material does not include a current Valve-endorsed SSD swap guide, so use a recent, model-specific teardown guide.
Recovery is also not harmless. Valve's recovery page describes factory reset, rollback, data erasure and reimaging, and warns that reimaging wipes the device. Back up first and make the recovery USB key before you open anything.
Check drive length and interface against the device's documentation before buying. ASUS's Ally models use M.2 2280 drives. That is not a safe assumption for a Deck, so confirm the supported size first.
A way to measure your own device
The original guide proposes a roughly 250-line Python harness. It runs four tests on SteamOS, Bazzite or Windows 11:
- sequential throughput
- queue-depth-one 4K random reads
- a long sustained write
- a small-file pass
It also uses hand-timed game loads to check whether the synthetic numbers predict what you feel. I would treat the method and its thresholds as sensible, not authoritative. Its thresholds are the author's rules of thumb, not standards: three runs per target, a 15% spread limit and a 50% write-drop "cliff". The code has also not been independently validated here, especially its Windows unbuffered-I/O path and its fallback to buffered I/O. Its sample figures come from a development PC and are not handheld results.
Some controls are good practice whatever tool you use:
- Use the same charger and power profile for every run.
- Close downloads and let the device cool between runs.
- Record the OS build with each result.
- Run each target several times and investigate large spreads.
- Verify a new card's real capacity with the f3 tools or H2testw before trusting it. Prefer the non-destructive f3write and f3read pair.
Be careful with destructive probing. A command like f3probe with --destructive wipes the device you name. Block device names vary, so never paste /dev/mmcblk0 without confirming it is your card.
If you'd rather use established tools, fio on Linux and Microsoft's DiskSpd on Windows can read with caching disabled for a cross-check.
What is and isn't verified
- Verified by manufacturers: the Deck's UHS-I slot, the Ally UHS-II readers, the X20's microSD Express slot, NTFS for the RC73 models, and the X20's October 15, 2026 availability at US$1,299 per ASUS's press release.
- Community guidance, not specifications: the ~100 MB/s Deck ceiling and reports of 2 TB cards working in the Deck.
- Unverified here: the source's claims about specific game-load gains, and any measured X20 Express-card benefit. No handheld benchmark results were supplied.
The takeaway is to identify your exact handheld and reader first. Format the card the way the maker specifies. Don't pay for card speed the slot can't use, and test with your own games before spending on a drive swap.