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Fix No Entry (not permitted) on boot from USB drive Mac OS X

I have come across this issue a few times, and always forget what causes it.

If you have a bootable USB flash drive with an OSX installer on it, but it fails to boot showing a not permitted icon this means that it found a boot partition and system to boot from, BUT it could not boot from it, this could be due to it being an incompatible system or more likely, a permissions error.

I have seen lots of good, and bad advice online about this issue, and NOWHERE does it mention the fix I found, so I thought I would document this to help people out.

Mount the USB drive on another mac computer and do a "Get Info", at the bottom it shows the Name of users and "everyone" and what read&write permissions they have for the drive, click the padlock to allow changes, and then change the permission for all users (especially "everyone") to be read&write instead of only "read", make sure "Ignore Ownership on this volume" is checked, then click the padlock to save it and close the window, eject the USB drive and it should now work (if you build the volume correctly in the first place).

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