

That's my experience of getting new ones. So, if you just bought an USB-stick, the chances are, that it is MBR-partitioned FAT32. Note: You should not use a version of Mac OS X that is earlier ("older") than the version your Mac shipped with. Contains an installation of Mac OS X 10.4.5 or later, or Mac OS X 10.5 or later, which is compatible with (or shipped with) the Mac that the USB device is connected to.

Has been formatted with a GUID partition type.
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Intel-based Macs support starting from an external USB storage device's volume that: The knowledge base article HT1948 states: All sticks should boot, but I have encountered some that didn't manage that. You'll need a bootable USB-stick with capacity of 8 GiB or more. I have 3 Macs to update and I don't want to download the huge package on all of them. The amount of downloading needed is easily 6+ GiB. Now that OS X El Capitan or version 11 is out, I'll do a refresh for the USB installation instructions.
