How can I boot via an external drive?

[iMore Question]

Got impatient when turning on file vault after Yosemite installation now machine just has circle with line in it and won't boot up. I have a backup on external drive but holding down option key doesn't show my external drive

 

[Just_Me_D]

It won't unless you've made the drive bootable. Take a look at the iMore Blog article via the following link, if you haven't already.

How to make a bootable Yosemite installer drive | iMore

 

[Mac Techie]

Hold command-r when booting to see bootable drives