Hi @SandPox,
Instead of manually mounting the image you can also try using resin config inject
like
resin config inject /path/to/config.json --drive /path/to/image.img
We recently updated our documentation to reflect the fact that resin config inject
can be used with both mounted SD cards and resinOS images in https://docs.resin.io/tools/cli/#config-inject-60-file-62- , and this should work cross-platform.