I'm going to check out command-option-R just for the heck of seeing what you ran into.
But regardless, before you upgrade I think it will pay you to run a "surface scan" on your external to eliminate a possible root cause of that prohibited sign, which just plain should not be happening. You don't want that drive to be compromised while you're into the upgrade process, because it's your backup.
Please copy & paste "diskutil list" (withOUT the quotes) into /System/Applications/Utilities/Terminal and post your results. (Don't forget to edit out your name if it's there.)
And now for dinner!
The new Great Equalizer is the SEND button.
In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire
