It's possible you used a machine specific 10.4 installer DVD? How you managed to install it though is harder to fathom? You may have been able to boot from it but it would have failed as soon as you tried to install it.
The installer DVD you want for Tiger is black coloured type rather than the gray coloured one.
I think your phrase ". . . loaded Tiger on a HD . . ." might be a clue as to how you actually "installed" Tiger? If you copied an existing OS onto an external hard drive and copied it over again, it won't work.