I dug out my A1200 last night and I forgot that I did get one partition working above 4 gig on it, but I was still unable to use more than about 12 gig on a 20 gig drive. I couldn't make that last partition any larger, and I couldn't add any partitions after the last one. But I have yet to try manually entering partition information.
It's my A2000 with 50 gig SCSI drive that really frustrates me. I can't get it to use more than 4 gig in any way shape or form. But again, I have yet to try manually entering partition information.