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2 Updated to avoid the implication that the ALT boot menu would only work for booting from external media. It will also show the OS X recovery partition if present
source | link

The Command key maps to the ALT key on a standard US English keyboard

Try holding down ALT + R and see if that works. Alternattively, if you have OS X media (CD or USB key)

Alternitavely, holding down ALT during EFI (when the screen is grey directly after power on) will present you with a menu of boot options. This should include the recovery partition on the hard drive, as well as any external media (OS X media for example) the system considers bootable

The Command key maps to the ALT key on a standard US English keyboard

Try holding down ALT + R and see if that works. Alternattively, if you have OS X media (CD or USB key) holding down ALT during EFI (when the screen is grey directly after power on) will present you with a menu of boot options.

The Command key maps to the ALT key on a standard US English keyboard

Try holding down ALT + R and see if that works.

Alternitavely, holding down ALT during EFI (when the screen is grey directly after power on) will present you with a menu of boot options. This should include the recovery partition on the hard drive, as well as any external media (OS X media for example) the system considers bootable

1
source | link

The Command key maps to the ALT key on a standard US English keyboard

Try holding down ALT + R and see if that works. Alternattively, if you have OS X media (CD or USB key) holding down ALT during EFI (when the screen is grey directly after power on) will present you with a menu of boot options.