So you're saying your 1TB drive is actually 900GB used with 300GB free? Surely you mean something like 700GB used and maybe 300GB free but then a TB isn't even 1000GB is it, it's something like 900GB and change.
So the old machine broke, and you plugged the hardware from the old machine into the new machine, OK?
Only displaying the folders up to F could be a problem with compatibility of the drive? Have you got a lot in the root folder of the drive? ISTR something a while back where you could only have something like a 500 items in the root of a drive, or maybe something less? Could it be that?
Have you tried it in another machine?
Have you tried connecting it back to the old machine which you attach to a spare screen/monitor or something?