The dual-450MHz G4 machines don't support drives larger than 137GB, due to their lack of 48-bit LBA capability -- while the drive will work, you'll be limited to a capacity of less than 137GB, unless you have the hard drive attached to a PCI ATA controller card that supports 48-bit LBA (most do). You might want to check the total formatted size of the hard drive and see if this is the case.
With that being said, I assume you got the jumpers right on each drive (one master, one slave), since you said "slave"... that'd be check number one. Second, I would check to make sure the cables are seated absolutely snug... just to get the basics covered.
Some hard drives have "alternate" jumper settings -- while they're rarely needed with a Macintosh, it may pay to check Seagate's site for alternate jumper settings for your particular model of drive that you could try.
Also, some Western Digital drives have two "master" settings -- one for "Master only" and one for "Master with Slave present". If your original hard drive is Western Digital, you may want to check this as well.