I installed Ubuntu on a 2010 Macbook, just to try it out. I now want to go back to either Sierra o High Sierra.
I created Install UBS sticks and I tried to boot from them by holding the Option Key. When I do that, a screen appears with the "typical" macOS icons for a HHD and a Removable Disc (orange icon). If I choose the HDD icon it eventually boots Ubuntu. If I choose the Instal USB drive, the screen goes apple grey with a "no parking" sign in a darker shade of grey (...).
I tried every combination I found on internet (cmd+R, cmd+option+R, cmd+option+shift+R, and some other maybe) but nothing makes the disk utility to pop up.
As an additional info, when I boot up, the typical Apple chime is there but no apple logo whatsoever...
Is there hope for something?