Well, after completely removing the partition table with diskpart before creating a new partition and formatting it I’m fairly certain there’s nothing left on the SD card. The first entry in the Groovesizer’s file browser is still a folder called system->1 (-> is actually a single arrow character that I can’t be bothered finding the alt code for).
Ok, managed to try it with another SD card (actually a MicroSD in an adaptor) and that has a System->1 folder too when browsing the root. The Groovesizer uses 8.3 naming convention, right? Is the arrow character being used in place of the ~ that DOS/Windows use? My computer is set to show hidden files so I’m not sure where this folder is hiding…
Solved! My instincts were right, the -> in ‘system->1’ was being used by the groovesizer in place of the ~ normally used in the 8.3 name format. The mystery folder was a ‘system volume information’ folder being placed there by Windows.
I had windows set to show hidden files and folders, but what I was missing was ‘hide protected operating system files’ being ticked.