OS X Installation USB not detected in Startup Disk selection

I have a MacBook Pro 8.2. Recently I have download Sierra and Yosemite, and am currently running El Capitan.

And I created a bootable USB for OS X Yosemite with DiskMaker X 6, in case Sierra goes wrong.

But when I try to check the USB, it's not on Startup Disk. I tried the option key on mac boot, but the USB is not there.

I'm using SanDisk Cruzer Glide Media, here the USB details.

In order to boot from an external disk it must have a GUID Partition Map. If your USB disk is not formatted with that you will need to reformat it accordingly.

1. Go to Disk Utility
2. Select "Erase"
3. Select GUID from the "Scheme" dropdown menu