Thank you, your help is greatly appreciated

! But tell me, if I buy a USB "adder", with 10 more USB ports, I can theorically add at least 10x 4 To HDD ?
BURST isn't really paying at the moment according to the calculator, but I'll stick to my idea.
A final question, maybe a bit hard to answer : what lifetime can I except from a new HDD running 24/7 BURST ?
Yes, just note that reading speed is limited to the connector. You might encounder problems running 10-20 harddrive from 1 usb on the motherboard. While not the perfect setup, it should be doable. You might also want to get a usb hub with external power.
According to backblaze, a new HDD last from 4 to 5 years and mining burst dosn't stress the HDD more then normal usage. I can't give a better answear, it's really the best answear I could find.
with usb i would split up the drives to several ports since if one drive on a bus times out it freezes all others.
depending on the setup you want (external usb drives with own psus) it may be cheaper and maybe more reliable to put some sata controllers into the rig and connect them to the systems psu (
).