Ay! Yep!
It is like that. Even if I plug in USB and mass storage mode should be enabled (calling osso-mm-umount), the MyDocs keeps mounted, so not accesible from PC.

I have investigated and it is definitely the loop mount of the image file.
I have booted with the entry in fstab automatically generated. But not mounted the image. This way enaböing USB works and MyDocs gets unmounted.
Once I mount the image file (creating /dev/loop0), the umount of MyDocs does not work anymore. Even if I umount the image file (/dev/loop0) in advance.

Only solution is 'umount MyDocs -l' manually.
This is lazy unmounting, also referenced in osso-mmc-umount, but not called.

I know how to workaround, but would like to know the reason (and correct solution).

Cheers.

--edit
Ay. Yes. I am sure. Also checked with lsof (nothing open) and fuser (do not know if output is correct).
And this behaviour I normally know only from Windoof (and/or DOS) ...