I'm unable to boot from a Mavericks install USB stick that works on all of my other macs. Anyone else have this problem? After starting the laptop with OPTION button pressed down I select the "Install Mavericks" USB stick to boot from, but instead of showing the Apple logo on the screen it just shows a "forbidden sign" (see attached photo) instantly. I've tried this with a USB stick and also on a USB hard drive. They work fine with my other macs (2013 MBA and a 2010 MBA) but not on this new rMBP. I would need this to fix my EFI partition table, which cannot be fixed from the Recovery Partition since it is on the same drive. It refuses to fix a drive when it is in use. Any hints for me? Maybe I need to finally buy a thunderbolt cable and then start the rMBP in target disk mode and then fix the partition from my 2013 MBA using Disk Utility? Thanks!