OK newer news!
Here is what happened:
http://eab.abime.net/showthread.php?t=94682
...and back to this topic, as now it is not a WinUAE issue.
So, the disk in my A4000 was luckily mine. But UNluckily, indeed has muFS installed.
At some point during boot, I get a "MultiUser Login Request", where it asks for login ID (I suspect which one I used) and then password (that I cannot in the slightest recall)... No way to cancel this.
I tried mounting my disk image (my actual hard disk is safely stored again - and even for the image I made a copy), as secondary disk to another working Workbench image. I get many icons in my desktop, which reminded me that the hard disk is split in many partitions!!! I can actually read many of them, but I suspect, not all.
And a boot options capture will verify me (check attached image).
What do I do now? Any ideas?
Maybe a way to hack this after all these years?
I suspect I boot from ADOS HDM, it loads muFS and transfers control to HDS which is muFS "formatted". I don't remember at all how this is done.
Also you can see more muFS partitions.
Totals: 1 NBS1 (??? wtf is that?) partition as "swap" (did I do all these? damn!), 5 FFS partitions, 4 muFS partitions...