Yeah, I just found out (thanks to my topic about a similar problem) that it was likely to be the kickstart. I don't have a KS 3.1 rom for the 600 unfortunately, only the 1200 3.1 rom. I'll try the hd in my 1200, see if it makes a difference, and if it does, well then I know I'll either need 3.1 roms or a faster hd for the 600.
BTW, my drive is a Seagate ST9190AG