Can't boot on any usb key.

Hi all,

Here's my issue:

iMac 24" (~2009)

Montain lion

When I try to boot on a usb key (Lion or Win7) on my imac the key isn't recognize. So I test the exact same key on my MBP 17" and I can easily boot on it. My usb ports are good, I can read the same usb key with my system, I have an issue only on boot.

I don't know if there's a link but my superdrive is broken and I can't eject the dvd in it (not a bootable one), I don't know if it can affect the boot.

Please let me know if you have any ideas !

Thanks a lot !!

Arnaud