Trying to boot OS X snow leopard from external hard drive, but not showing up as a startup disk

Question

As the title says, I'm trying to boot from an external hard drive, but for whatever reason it isn't showing up under System Preferences>Startup Disk. I saved about 211 GB of data onto my external hard drive, which is a "Seagate Backup Plus, Portable Drive, 1 TB."

This following is all I have done. I used time machine to copy my Mac's hard drive onto the external hard drive, and when doing so it a said I needed to erase everything on the Seagate in order for it to be correctly formatted. Of course I did this, and the data was copied over the next few hours. Now it seems to have everything on there, but I just want to boot from this hard drive instead of the internal one, but it won't show up as a startup disk!

I'm upgrading my Snow Leapard to Mavericks, so I want to make sure I have a bootable version of my internal hard drive before I do this. Any other advice on accomplishing this is appreciated as well, thanks!

Answer 1

A time machine backup isn´t bootable. If you want to create a bootable backup from your internal hard drive please use Carbon copy cloner.

And another hint: If you want to check if a external hard drive are bootable: plug in your hard drive and restart your mac. If the apple sign appear press and keep pressed key "ALT" and wait. Now it shows you all boot options.