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macrumors newbie
Original poster
May 29, 2016
Help. My USB isn't being found when I hold down the Alt key on setup.
The only things comes up is my HDD and recovery


macrumors G3
Sep 21, 2010
1) Have you verified that it's bootable on another computer? The process for making a USB thumbdrive that is actually bootable is prone to failure.
2) Did you connect it directly (i.e. not through a hub)?
3) Did you wait a while? When the initial list of bootable partitions comes up, the Mac is actually still polling for additional bootable partitions which may appear later.


macrumors Penryn
Feb 20, 2009
Some other things to try:

Boot from the INTERNAL drive.
Connect the USB external.
Go to the Startup Disk preference pane.
Does the external drive show up?
If it doesn't, the Mac doesn't "recognize it" as being bootable.
If it DOES show up, select it and try "restart".
What happens next?

Also, try this:
Power down -- all the way off.
Disconnect the external drive but "keep it close by".
Reboot, hold down the option key to bring up the startup manager.
Once startup manager appears, NOW CONNECT the external drive.
Wait a little bit.
Does it now show up as a bootable option?

Final thought:
How did you CREATE the bootable external drive?
Did you install a "clean copy" of the OS?
Did you clone your internal drive to it (using CarbonCopyCloner)?


macrumors 6502a
Aug 13, 2012
Long Island, NY USA
I'm having a very similar problem. I have a Sonnet USB 3 card installed and an external drive with OS 10.8.5. This drive will boot from the native USB 2 ports but not from the Sonnet USB 3 card, so the problem seems to be with USB 3 or the Sonnet card, which does not need drivers. I noticed that the attached drive's led goes dark during boot and then comes on again, as if the card is performing a self test. Maybe this could be stopping it from booting the system? At any rate, I'd like to try another card if someone knows of one which a Mac can actually boot from.
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