I installed 9.0 again last night on 40GB seagate drive with the same result.
I also installed FreeBSD 8.3 on one of the WD drives with no problems. I'm posting from Firefox 10 with openbox on FreeBSD 8.3. I will have another go at 9.0 this weekend - I notice that 9.0 uses a newer installer called bsdinstall, whereas 8.3 uses sysinstall.
//edit: reinstalled 9.0 again, taking great care - same result, tried resetting factory defaults to BIOS - same. Anyway I am happy with 8.3, so going to stick with that for now.
Thanks for helping this noob everyone.