Thank you BradHeff and critter for your quick replies. Actually, I installed it that way, BradHeff, and it didn't work out. No idea why. As for the boot option, critter, I use the F12 key, in order to access the boot options, but the funny thing is that sometimes my USB stick is detected, sometimes not. So it occurs quite often that I cannot boot using Solus because the USB key is not detected. I am using a Verbatim key, maybe it's why. And yes, I used 2 USB keys during the install process.
As it was complicated to deal with the whole thing, I decided to work mainly under Windows, using Solus only to transfer files from an external drive, which was formatted under macintosh. Even in that case, it doesn't work really well. So would you get any advice in order Solus works better, when booting with the USB stick? That would be nice if the USB key would be always detected by the boot system and that when I shut down, the process is normal (and not really slow).
Thanks again for your help, have a great day.