I read somewhere on the forum that the transfer app doesn’t work with OS v1.46B on the Rytm. I can’t remember exactly where it was posted, so you will have to scan the forum to verify that. I too have a Rytm mk1, so I looked this up a while back and that’s what I found out. I think you might need the next non BETA OS before v1.46B to use the transfer app with it.
Personally I am only transferring samples that I have tailored specially to blend and tweak with the AR’s internal synth engine, so I am only transferring a handful of samples at the most in one go per project. I am in no rush to transfer large batches of samples in one go, so I will just wait until an OS that is compatible with the transfer app (or visa versa) is released rather than reverting to an earlier OS and I am not to sure whether or not reverting to an earlier OS would be safe anyway.