Steam Machine internal microSD slot keeps exposing card as /dev/sda but never creates /dev/sda1

Your system information

• Steam client version: (not checked, can add if needed)
• SteamOS version: 3.8.16 (BUILD_ID=20260716.1, stable branch)
• Kernel: 6.16.12-valve24.5-1-neptune-616-gb2f7cfe85e45
• Opted into Steam client beta?: No
• Opted into SteamOS beta?: No
• Have you checked for updates in Settings > System?: Yes

Note: This is the Steam Machine's built-in microSD slot. The kernel exposes it through a USB storage path (Realtek 0bda:0316) instead of an mmcblk device.

Expected behavior:
When I create a partition table and partition on the internal microSD card, SteamOS should create a partition device like /dev/sda1 that I can format and mount normally.

What did happen:
SteamOS only ever shows /dev/sda with no partition node (sda1). The card is detected, raw reads/writes succeed, and partition tools sometimes report a valid partition table, but the kernel never creates /dev/sda1.

Key observations:

• lsblk shows only sda, never sda1
• partprobe -s /dev/sda reports msdos partitions exist
• partx -a /dev/sda fails with failed to read partition table
• blkid /dev/sda returns nothing after formatting attempts
• Raw writes to /dev/sda succeed (tested with dd)
• The card was formatted in a Samsung phone, then reinserted into the Steam Machine. The Machine still detected only /dev/sda and no partition node appeared.
• This removes ambiguity about whether the card itself is defective.

udev info shows the internal slot uses USB storage, not mmcblk:

(deck@steamdeck ~)$ sudo udevadm info --query=all --name=/dev/sda | grep ID_
E: ID_BUS=usb
E: ID_MODEL=SD_MMC_CRW
E: ID_MODEL_ID=0316
E: ID_VENDOR=Generic-
E: ID_VENDOR_ID=0bda
E: ID_USB_DRIVER=usb-storage

lsblk confirms USB transport:

(deck@steamdeck ~)$ lsblk -o NAME,MODEL,VENDOR,SERIAL,TRAN,SIZE
NAME        MODEL                     VENDOR   SERIAL            TRAN     SIZE
sda         SD/MMC CRW                Generic- 16140211162022000 usb    366.8G
zram0                                                                     7.6G
nvme0n1     KINGSTON OM3SGP4512K2-A00          50026B7283DE94F6  nvme   476.9G
├─nvme0n1p1                                                      nvme     256M
├─nvme0n1p2                                                      nvme      64M
├─nvme0n1p3                                                      nvme      64M
├─nvme0n1p4                                                      nvme       5G
├─nvme0n1p5                                                      nvme       5G
├─nvme0n1p6                                                      nvme     256M
├─nvme0n1p7                                                      nvme     256M
└─nvme0n1p8                                                      nvme   466.1G

sgdisk shows no partitions despite GPT header existing:

(deck@steamdeck ~)$ sudo sgdisk -p /dev/sda
Creating new GPT entries in memory.
Disk /dev/sda: 769277952 sectors, 366.8 GiB
Model: SD/MMC CRW      
Sector size (logical/physical): 512/512 bytes
Disk identifier (GUID): 8E0E2707-7CE6-4E8F-B43D-4B2805DD4691
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 769277918
Partitions will be aligned on 2048-sector boundaries
Total free space is 769277885 sectors (366.8 GiB)

Number  Start (sector)    End (sector)  Size       Code  Name

Raw writes succeed, proving the disk is writable:

printf "HELLOTEST" | sudo dd of=/dev/sda bs=1 seek=4096 conv=fsync
sudo dd if=/dev/sda bs=1 skip=4096 count=9 status=none
# Output: HELLOTEST

System info:

(deck@steamdeck ~)$ cat /etc/os-release
NAME="SteamOS"
PRETTY_NAME="SteamOS"
VERSION_CODENAME=holo
ID=steamos
ID_LIKE=arch
ANSI_COLOR="1;35"
HOME_URL="https://www.steampowered.com/"
DOCUMENTATION_URL="https://support.steampowered.com/"
SUPPORT_URL="https://support.steampowered.com/"
BUG_REPORT_URL="https://support.steampowered.com/"
LOGO=steamos
VARIANT_ID=steamdeck
VERSION_ID=3.8.16
BUILD_ID=20260716.1
STEAMOS_DEFAULT_UPDATE_BRANCH=stable

(deck@steamdeck ~)$ udisksctl status
MODEL                     REVISION  SERIAL               DEVICE
--------------------------------------------------------------------------
KINGSTON OM3SGP4512K2-A00 SDW02105  50026B7283DE94F6     nvme0n1 
Generic- SD/MMC CRW       1.00      16140211162022000    sda     

Steps for reproducing this issue:

1. Insert a microSD card into the Steam Machine's built-in slot
2. Observe that it appears as /dev/sda with TRAN=usb (not as /dev/mmcblk*)
3. Use fdisk or sgdisk to create a new partition table and partition
4. Run lsblk and observe that no sda1 appears
5. Try partx -a /dev/sda and observe it fails with failed to read partition table
6. Format the card in a phone (Samsung), reinsert, and observe the same issue persists

Related issues

Adding states of cases at time I messaged, two are still open, including #1208 which still returned at #1502

• Closed - SD card not being automatically mounted after updating to 3.5.5
• Open - SD card not being automatically mounted after updating to 3.5.19
• Closed - SD Card not working on Gaming Mode and not detected on Desktop Mode (SteamOS 3.5.17)
• Open - Steam deck not detecting SD Card
• Closed - (SteamOS 3.6/3.7) unable to mount any external storage devices

Start by reproducing the internal-slot behavior with the reported SteamOS 3.8.16 kernel, then inspect the udevadm and lsblk output for the USB storage path (Realtek 0bda:0316). Compare partition discovery using fdisk or sgdisk with partprobe and partx. Done means a valid partition table on /dev/sda results in a usable /dev/sda1 node.