I was getting recursive error 6 & 7 on 2 drives for no reason that I could find. I took off the top of the drives, compared the drive mechanisms and could not find anything very strange.
Then I realised that neither of the drives were precisely at the start points. the pickup arm was not at its start point and the cassette mechanism was not fully at its start point either. I carefully wound the cogs of both mechanism back to their respective start points.
After restarting the drives, placing scratch cassettes in the mouths, I managed to get into the mechanical test menu and launch load tests and full tests. Now both drives are working fine.
All I can deduce is that the drives are mechanically very sensitive to position and struggle with incoherences.
I hope this helps someone.