Issues with 6th gen not recognising Rockbox

I have installed Rockbox on multiple ipods - 3rd, 4th, 5th, 6th gen for example - and never had this issue before.

The Rockbox installer autodetects it as a 6th gen and the installation seems to go flawlessly, but the ipod just boots into stock OS every time I restart it. I have tried daily build, stable build, reinstalling bootloader, not reinstalling bootloader, copying .rockbox folder into route directory of ipod, all i can think of. Ipod shows up fine in File Explorer and MusicBee, I can copy stuff to it - but no Rockbox. It has the original Hard Drive so no SD card issues. Itunes recognises it and it's functioning fine in OS so no issues with HD I don't think. Menu+Select = stock OS. Menu held down + select = stock OS (as expected).

I'm close to stumped - any ideas?