To quote from the link below:

Bootability comes down to a few simple rules:

1. The hard drive enclosure must support booting a Macintosh (applies to external hard drives only)
2. The computer must support booting from the interface used to attach the hard drive (e.g. USB, Firewire, eSATA)
3. The computer must support booting from the hard drive’s partition format (e.g. APM vs GPT vs MBR)
4. The cloned filesystem must have all the required components of Mac OS X
5. The cloned operating system must be properly “blessed”

See the complete info at:

http://www.bombich.com/software/docs/CCC/en.lproj/troubleshooting/what-makes-a-volume-bootable.html