Just wanted to say a big thankyou to everyone who has posted all these useful comments on here and wanted to share my results. I got the ide adaptor which absorb mentioned in the previous post (took about 2 weeks to come in the post) and the hdd I used was 1TB WD10EADS Caviar Green 32MB.
Using xboxhdm_1.9 I formatted the disk in the usual way and then used xbpartition to distribute the storage evenly (although it said I only had something like 475gb in drive F and G - thought it should have been nearer 495 but not to worry). I have tested it on the following xboxes:
xbox v1.0 - working although occasionally G drive isn't available and there seems to be a 50/50 success rate of booting up, if it doesn't boot it will either be an error 13 or 21.
xbox v1.1 - working really well. Only slight problem is that I occasionally get an error 13 or 21 on boot but I'd say 4 out of 5 times it boots perfect.
xbox v1.6 - same as v1.1 (although I didn't thoroughly test it, but the few times I did it seemed to be the same results as v1.1).
All the above results were based on having the lid off, if I screw the lid on then I cannnot get it to boot - think it's error 13. I am currently using v1.1 with lid placed on but not fully clipped down into place. Oh and I don't know if it helped or not but I did stick some cloth tape between the hdd and adaptor so the solder points wouldn't get earthed. Hope that helps someone out there.