Windows – What’s faster? Moving files from one drive to another, or moving files in the same drive

"It depends." All other things being equal, platter density does directly correlate to an increased sequential transfer rate.

However, you need to know what the platter density is. If that 400 GB drive has 2 x 200 GB platters and the 80 GB drive uses 1 short stroked 200 GB platter, then the 80 GB drive would be faster overall (because the average seek time will be smaller).

The extra drive space may end up being useful in some other way over the life of the system though. Unless you're looking at buying thousands of systems were $13/ea is a budget breaker, I'd opt for the larger hard drive immediately. Otherwise if you end up wanting or needing a larger drive, you're looking at not just the expenses of the drives themselves, but of your time spent swapping the drives and imaging the contents.

Personally, I wouldn't install such a small drive in a new system unless there were other considerations coming into play. (e.g. It's an SSD, budget sensitivity, etc.)

Each partition has it's own, independent file system. If you move a file within a partition, the file system can simply rewrite the reference to that file.

But when moving a file between partitions, the file system on the destination partition doesn't know anything about the new file - there is no reference to adjust. Furthermore, each partition will have it's own physical area of the drive to itself, so the file must be copied to the new partition, then deleted from the old partition.

Copying large files in this manner is often slow, probably even noticeably for some SSDs, because the drive is thrashing between reading one area and writing to another.