Apple - Macbook Pro not detecting bootable USB with Yosemite

You might try resetting the pram on your mac. I understand that startup disk settings are stored there.

If the disk is bootable on other Macs, it stands to reason that it should boot on this one.

You can reset the pram by doing the following:

1. Shut down the Mac.
2. Locate the following four keys: option, command, p and r. You'll need them in the next step.
3. Power on the Mac, then immediately hold option, command, p and r. You need to do this before the grey screen appears.
4. Hold down the keys until you hear the startup sound twice, then release them.
5. Plug in the bootable USB stick, restart the Mac, and hold option.

I can't guarantee that it'll work, but it won't hurt, so it's worth a try.

I just ran into this as well. When booting from a USB disk that seemed to be bootable, it would display a "no" symbol ( e.g. ⃠ ...) after trying to boot into it and would then boot into the OS. I tried wulfderay's suggestion (the currently voted answer) to reset the pram and it didn't help.

I came across this installation tutorial and figured out that #5 in Step 1 of creating the drive had been wrong in my case. It's important to create the partition as "GUID Partition Table" rather than either of the other two. Mine was previously "Master Boot Record" as a partition scheme.

I'm guessing this could be the edge case that a few of us have run into. I confirmed the SHA of my Yosemite download and used the same one to re-create the bootable USB, so this was the only difference.