There is a limitation in Mac OS 9.x that will not recognize a system on a volume larger than 190GB.
Backup any data you do not want to lose. Reinitialize the drive and partition it. Be sure that there is at least one volume smaller than 190GB for Mac OS 9.x to be installed on. Once this is done, there will be no problem booting.
Althought the TSATA supports booting from an attached drive, it does not support target disk mode when booting from an attached drive. Try connecting the two computers via ethernet to exchange files.