I upgraded my Valve Steam Deck from 1TB (2x512GB) to 2TB (2x1TB) using a 1TB Samsung NVMe 4 M.2 SSD and a 1TB Sandisk Extreme Pro microSDXC.

The Steam Deck uses ext4 as its filesystem, which you can mount under Linux natively, or mount under Windows and MacOS using the proper drivers or kernel extension.  While there’s multiple solutions like MacFuse for the Mac, I prefer Paragon Software’s LinuxFS for Windows and extFS for MacOS to provide ext4 filesystem support under Windows and MacOS respectively.

If you’re running Apple Silicon (M1 or M2) rather than Intel on your Mac, you’ll need to boot into Recovery Mode by rebooting your Mac, press and hold the fingerprint scanner / power button until it displays the “Loading Startup Options” screen…

1. Press and hold the power button until you see “Loading Startup Options”
2. Click Options.
3. Click Continue.
4. In the menu bar at the top, Select Utilities > Startup Security Utility.
5. Select the startup disk.
6. Click Security Policy.
7. Select Reduced Security.
8. Reboot and continue with Paragon Software’s extFS for MacOS installation.

Once you can mount ext4 filesystems on the Mac, you can use your Mac to either `dd` or copy (using Finder) your steamapps and gamesaves from the old storage devices to the new (SSD or microSDXC).

Since the SteamOS 3.x (Holo) is effectively Arch Linux, you can also switch to Desktop Mode, enabled sshd and either cp or scp files to/from your Steam Deck using your local secure shell or Terminal.  This is very useful for absolutely everything, allowing remote management and copying to/from your Steam Deck.