OK, here are some new results.
I found at last how to configure HxC as my drive 0 (or A, or boot drive) and my Microdisc as my drive 1 (or B, or slave drive).
With this configuration, I booted up successfully with some .DSK images I have (such as SEDORIC or games)
Here are the settings I used (HcX software drive letter is A) :
But unfortunately I never be able to make a backup of my sedoric .DSK onto my actual 3" diskette
Each time, when backup is finished, when I tried "DIR B", I got a "Undefined format error" message...
I tried several combination of DTRACK command but it was useless.
Then I understood that at the end Microdisc is in fact a 1 sided disc drive (you have to flip manually the disc to access side 2 !!).
Whatever the TRACK or DTRACK command I set (even if the SYS command says Drive B is 1 side) the BACKUP command tried to copy the 2 sides of the SEDORIC disc without asking me to flip the 3" to drive B (so the tool rewrite the side 1 tracks with side 2 tracks). Perhaps I missed something ?
To proof my idea, I reseted the computer just when BACKUP was finished with the last track of the first side. Then I rebooted (on HxC) and performed a DIR B and... tada !! This time I got a list of files, not an error.
So now regarding what I explain here, did I missed a (special) command somewhere or is there an issue/limitation regarding HxC ? In other words, would it be possible I have some "so-called" 2 sided .DSK file that actually comes from a 1+1 side (you understand what I mean...) actual disc ? Would it required somewhere a "flip disc side" HxC command ?