The scsi.device of your (original) Kickstart ROM will never support partitions with more than 4GB so you must keep your boot partition below this barrier.
You can replace the scsi.device later in your RAM, but at boot time you must live with the scsi.device from your Kickstart ROM.
There are of course ways to burn your own Kickstart EPROM with more recent device drivers, but that's maybe too complicated for you.
Keep your boot partition below 4Gig and install some extra software (scsi.device, FFS, NSD etc.) and you will be fine for any size hard drive.
http://os.amigaworld.de/index.php?lang=de&page=37
