OS X V10.4 :: Cannot Boot Up From External With Snow Leopard Installed?

I've got an iMac Intel Core 2 Duo running Tiger 10.4.11.I've successfully installed Snow Leopard 10.6.3 onto an external drive (or so the screen said).My mac recognizes the external drive as a start-up disk and allows me to select it as such. But upon restart, machine comes up in Tiger.My external drive is brand new, OWC Mercury Elite AL Pro using a fire-wire connection. After installation but two failed attempts at start-up, I ran first aid on the external and no repairs were necessary. Using System Prefs I selected the external as my start-up disk, hit Re-start, but machine still restarts from Tiger 

The only other piece of information that may be vital, that I can think of, is that I misunderstood the "Mac readiness" of the drive when I took it out of the box. I first erased it (unnecessarily). Then I discovered this mistake, and made a point to re-format it using the GUID partition setting. Computer claimed that installation was successful upon installing Snow Leopard.But now, machine will not start up in Snow Leopard.

Info:
iMac (20-inch Mid 2007), Mac OS X (10.4.11), 20-inch, 4 GB 667 MHz DDR2 SDRAM