I made an OS X 10.6 USB stick and I used it to succesfully install OS X on several Mac, including a very old Mac Mini and a MacBook Pro.
However I've got a problem: on my girlfriend's Mac it simply doesn't work.
When I hold ALT when the system boots up, I can see the harddrive icon (so I take it that pressing ALT does indeed work) and hence decide to boot from the harddrive, but I don't see the "Recovery HD" nor the USB stick / "EFI boot" icons. The only choice is get is "harddrive".
I tried again on my Mac Mini and surely enough: I can hold ALT and I see all the icons.
I also tried again on my MacBookPro (very old 17" MBP, the first 17" one if I remember correctly) and once again, I can hold ALT and I see the "EFI boot" icon.
Why doesn't my girlfriend's MacBook recognize the USB stick (and hence prevents my from reinstalling the system)?
The strange thing is that as far as I can tell my girlfriend's MacBook is more recent than both my MacMini and my MacBook Pro.