One other weird thing is this, the PC doesn't like my USB IDE adaptor and does not boot with it connected, but I did get it to work once by plugging it in at just the right moment in the boot process and a drive which I had written 0xFFFF over the superblock was connected, when I read it I am sure I got 0xFFFF back, but when connected to the IDE interface I get 0x7FFF. The thing is I also get 0x7FFF when I read it through the USB adaptor on the Mac so I don't know what to think. I have been unable to get the USB adaptor to work a second time so had sort of disregarded the one off event.

I believe the results are the same regardless of drive and which IDE channel I use, I am half way through writing zeros over the whole of one of the 500Gb drives atm so can't retest to be absolutely certain.

As I said, it was 4 drives which IIRC were RAID 0+1 array. The machine I am doing this on does not support RAID, I am mounting one drive at a time. The 1.6Tb number must be coming off the drive somehow but I can't fathom it.

Sorry, this is with the drive connected via IDE and the PC booted from a USB memory stick with the gparted live iso image as suggested by John Kintzele, but for some reason his answer has disappeared.