A number of questions ~ what type of Tiger install disc/s are you using, a black full retail install or a silver grey disc? Generally the silver grey discs will not work as they are for a specific model and not able to be installed on other machines.
Did you boot from the Tiger disc, if possible, holding down 'C' immediately after the chimes and selecting the external drive?
Have you formatted the external drive Mac OS Extended (Journalled) partitioned the drive in Disk Utility, even if set up for a single partition, and gone to Options under Partition and set it up as Apple Partition Map?
