How to reinstall from an USB stick on a MacBook or iMac? 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.
If her MacBook was shipped with Lion, you couldn't be able to boot with a Snow Leopard stick. Apple Knowledge Base states that you shouldn't use earlier versions of OS X:


  Note: You should not use a version of Mac OS X that is earlier
  ("older") than the version your Mac shipped with.


If this is the case, you have to create a bootable Lion stick.