EDIT: Also, make sure that the HD20 is plugged into the mac first, then the dongle behind the HD20. (I would test with my own HD20, but its drive is not responsive.)
I did some testing and the results are strange.
I tested first with these two setups:
512Ke – HD20 with Sys 6.0.7+driver 1.7.1 – Dongle+Rapport Drive – External 1,44 Mb floppy drive (A9M0106)*
Plus – HD20 with Sys 6.0.7+driver 1.7.1 – Dongle+Rapport Drive – External 1,44 Mb floppy drive (A9M0106)*
Both work flowless, even after several startups. And it gives the benefits of a working 1,44 Mb floppy drive as well.
From top to bottom: HD20 drive – Rapport 2,4 Mb floppy – External 1,44 Mb floppy – 800K internal floppy
Than came the strange part. I replaced the Plus for an original SE (non FDHD) and started it up with the inside harddisk (BlueSCSI). Put the driver 1.7.1 in the control panel of the system folder (Sys 7.1), turned it off and connected the HD20+Rapport+1,44 drive. During the next startup, as soon as the Rapport driver should load, mostly the startup freezes and sometimes the driver loads with the icon showing "not installed".
The same happened when I managed to startup the SE from the HD20: freeze or "not installed". While the same exact setup with a Plus or 512Ke works fine. I repeated the same setting with an SE FDHD and a Classic, both with the same results.
So the question is: what caused this difference in behavour? It seems to me that this setup only works with the Plus and 512Ke and most likely on a 512K with a startup floppy containing the HD20 extension. I can test this, but I have to swap the 512Ke-ROMs for 512K-ROMs and make a 400K startup floppy with the HD20 extension on it.
I also came to the conclusion that the BMOW Floppy Emu in HD20-modus won't work together with a Rapport drive. The Floppy Emu needs to be plugged in first, but it does not have a port to connect more devices. Connecting after the Rapport drive give always a freeze at startup when the Rapport driver should load.
* original 800K drive just swapt for a 1,44 Mb drive