Hi guys, I just wanted to tell you that I managed to fix the Apple HD 20 SC. Here's what happened:
the HD had a pre-installed System inside (I don't know which version and I don't know on which machine it performed the installation). It was therefore a bootable HD and (oddly), any selection of the SCSI-ID was not taken into account and made me boot always and only from the HD 20 SC. So I put a System 6.0.8 bootable from Floppy on the Macintosh SE to boot from floppy (the Macinotsh gives priority to the floppy to boot, then to external HD and lastly to internal HD. At least for that's me). So, started the System from Floppy I was able to see also the HD 20 SC, and from here I removed the installed system. Seeing no boot disk from the HD 20 SC, all my macintos manage to see it as secondary.
I mainly needed it as HD for my plus. So I connected it to my macintosh plus and started from floppy (System 6.0.8), from here I installed the 5 floppies on the HD 20 SC. The reason why my Plus didn't see the HD 20 SC was because you actually need to turn on the HD 20 SC first and then the Plus; my problem was that the plus was trying to boot the system from the HD 20 SC, and it was definitely not compatible, that's why it kept restarting.
Thank you very much for all for your interest