TLDR: Use Windows

My only success of using the second SD card for ROMS was to use Rufus (as per Windows guides) to format the SD card, then try that way. Attemping to format and use the card using the Disk Utility in MacOS failed every time.

Backstory

I purchased a R36s retro handheld console from AliExpress recently (at a bargain of £11 during a sale), and beforehand I had read up on the device - mostly on Redit.

One suggestion of using the device is to make use of the dual SD card slots. One for the OS and one for ROMS. This meant that if ever upgrading or repairing the OS, then there’s less of a chance of losing games.

I opted for two Samsung Evo Plus cards (see this compatibility list of cards), one (OS) with 64GB and the other (ROMS) 128GB.

The handheld and the 128GB SD card arrived the same day - which sounds good, but I wanted the 128GB for ROMS. Eager to play some reto games (mostly SNES to start), I got on with using the 128GB card straight away and all was fine.

64GB card turned up a few days later - turns out 64GB cards aren’t as easy to come by now and £ per GB means the 64GB is actually more expensive.

I followed a couple of guides on formatting the SD card for the second slot, but nothing seemed to work. Formatting the card was all fine (trying all the file formats I could using the Disk Utility), and switching the R36s to dual card mode went fine - no errors or warnings on the display at all. In fact, using the file manager/browser on the R36s I could see the game system folders that the process creates, but inserting the SD card into the MacBook I could see nothing.

I tried for a couple of days to get the card to work, then gave in and tried my (very) old gaming PC. I followed this guide on using Rufus in Windows to format the card. When I inserted the cad, Windows was unable to read it and it turned out there were multiple partitions on the card. I formatted the card and deleted the partitions then went back to Rufus. Once formatted using Rufus, I put the card into the device and swtiched back to 1 card setup (if the device is in a 2 card setup then it needs the second cart to boot up). Restart the device then change to use 2 card setup. Again, the process had no errors or warnings.

Card back into the Mac (where the ROMs were backed up to) and tadaa! System folders!

I then copied the backup ROMs folder to the folders on the card. Inserted the card into the device and sorted. Job done.