Contents not deleted as have all my save games on there and the guy building it said the new one would be recognised as "C" drive and I would be able to access everything still on the old one!
Which are you referring to as "the new one"?
The system came with the Intel 665p, Windows installed on that. Correct?
You have a WD Blue from a different system, that had/has an OS on it, from that older system?
That WD Blue will NOT be seen as the C drive.
Whatever drive the system boots from will be the C. Other drives and partitions will have different drive letters.
Hoping that this new system boots from the old WD Blue with the old OS is simply wishful thinking.
If your builder guy said it WILL boot up from that..or it would be seen as a SECOND C drive...he is woefully mistaken.
Additionally:
You can access the contents of the old drive. BUT, anything contained in the Libraries /Documents/Music/Pics/etc., will involve TakeOwnership. Those library folders carry permissions from the old user in the old system.
Also, any applications on that drive Will Not Work.
Steam games you can probably make to work. But any regular application...no.