I’m not sure, you do talk about the same thing. 
As far as I know, under HassOS, the boot from SSD is not supported. This is only supported by installing Raspian and afterwards install HA as supervised (so HA is “running” your OS, but it’s not HassOS) or you install Rasbpian and afterwards Docker with Hass.IO.
Tbh I’m a little surprised HassOS is working for you @ericpignet , not installed as supervised. Are you sure, you didn’t install Rasbpian and afterwards the supervised installation?
To sum it up, a plain HassOS will not run from SSD on a Pi3. You need to install Rasbpian and afterwards the installation method you want. I for one choose the Docker installation method. It is clean, easy to handle and is running nicely. And it is Hass.IO in the end, with all the added benefit of addOns and so on.
EDIT: I found the thread, where I read about problems with Pi3 and SSD. Check it out: