I'm trying to install a 20GB hard drive on a 1200 with accelerator board; I have 3.0 kickstart and I'm trying to use FastROMAtaPatch for the scsi.device and fixhdsize, but the maximum amout is always 8GB!! 
What can I do??
The answer is in the posts above.
Run IDEFIX, it contains a number of patches to Scsi.device that are sessential, and run a better File system than FFS, like SPS or something.
I have a 20GIG in my A1200.
But AFAIK, Workbench 3.9+ you do not need IDEFIX. BUT !!!!!!!!!!, to install 3.9, you need a CDROM, to access the disk, and since you do not get those drivers from the CDROM until you have a cdrom drive working.... hmmmm..
Use IDEFIX, install 3.9, delete IDEFIX. but as you have kickstart 3.0, you are running Workbench 3.0/3.1 yes ?, IDEfix for you my buddy.