I discarded a large HDD some time ago thinking it to be damaged and have been using a much smaller disc, I have now discovered that the original disc was not damaged (another story altogether) and have replaced it in my computer together with the smaller one. Both discs have Windows XP with SP2 on them but they are not the same system (different validation keys etc). Could I use something like Acronis to transfer the contents of the smaller disk to the large one and would it overwrite the large disk contents or would I have to remove everything from the large disk prior to transfer?
You would be better formatting the big drive first as the original data will cause conflicts with what you're replacing it with, and the effect would that your computer would ask to be reactivated every 3 to 4 weeks. I had the same problem with the same thing that you're doing, in the end I had to contact Microsoft UK and they put me in touch with a team from India to sort the problem out.