You're trying to put 300GB drives in your Amiga?
3.9 has fewer limitations than previous versions, but even if you use the latest HDToolbox and
PFS or
SFS file system, you're probably still going to have some issues creating partitions out beyond 128GB. That's assuming your controller supports them and you have the right adapters to physically plug them in.
IMHO unless you have a pressing need for that much storage you might be better off starting a little smaller until you've acquired more experience? But what the heck, you only live once, right? Go for it.