This is a hypothetical situation, but understanding it could really help me out.
If two PCs are exactly the same.. have the same parts same brands same models.. same OS.. same drivers and programs.. put together at the same time and were used the same amount of time. but one is copying (from partition to partition on the same HDD) on Windows (7) at 40mb/s and the other at 55mb/s..
Assuming that we're sure that all parts of the two PCs are performing fully and normally.. but we're not sure about the HHDs..
What could be responsible for this performance difference?? could this be a hardware failure in the lower performance HDD, or this considered a usual individual difference between these two hard drives and not a defect in this HDD??
About the only thing I can suggest for such a difference if everything else is truly the same is that the slower one has much more fragmentation of files on its hard disk Partitions. That would slow down both finding all the parts of each file on the first partition, and finding empty sectors to use on the second partition in which the file can be written.