Unable to Boot from External USB Drive

I have an external USB drive, and used Carbon Copy Cloner to make a copy of my internal SSD. However, when I reboot and hold down the option key, the external drive does not appear as a choice (only my main drive and boot camp partition). I recall this working on my old MacBook Air. Has anyone run into issues either with CCC or the 2011 MacBook Air in this regard? I'd like to set up the external drive to use for testing/beta purposes.

It is formatted HFS+ and has the GUID partition table. I also created a restore partition on the external drive using CCC.

simsaladimbamba said:

Does it show up in System Preferences > Startup Disk?

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I think it may be a bad drive. Sometimes it shows up and sometimes it doesn't. Also, when I mounted it, it gave me the "drive cannot be repaired" error message. I reformatted it and cloned it again using CCC, and it worked once, but then gave me the error message again. It is an Intel 320 SSD, which is normally pretty reliable, but perhaps I happened to get one that slipped through QA.