Helping people with computers... one answer at a time.
Surprisingly, it just might be faster to copy to an empty disk. But the reason for this is a little more complex than simply the amount of data on a drive.
Is it true that writing to a disk drive having lots of empty space is faster than writing to one having less space? Suppose I have 1 GB of data to write and I have felt it is faster to write this data on to a disk drive having a space of 10 GB than on a drive having space of 5 GB? But I'm an electronics guy. Theoretically, the given amount of data should give the same amount of time to be written irrespective of the remaining space on the drive. Of course, given the empty space on the disks are greater than the size of the files being written to it.
In this excerpt from Answercast #18, I look at the time it takes to copy files onto a disk with lots of space compared to copying the same files to a more full disk.
This is actually a really interesting question. I've never thought about this way. The surprising answer is maybe. It could be faster on an emptier drive.
So the concept here is that you have a drive. It has say, 20 GB of free space and you write 5 GB of data to it and it takes "X" amount of time. You go to a different drive, repeat exactly the same kind of a thing except that the different drive has only 10 GB of space available. The 5 GB copy is just exactly the same copy you did before.
It feels like it takes longer.
This isn't really about the amount of free space on the drive. What it kind-of boils down to is (of all things) fragmentation.
A drive that is full, or close to being full, is more difficult for the defragging tool to defrag. In fact, sometimes, defragmenting of many files simply won't happen if there isn't really enough room.
What happens then is on a drive that is more full than another, files tend to be more fragmented. Conversely, the free space into which you'll be writing your new file is also more fragmented.
What that means is that the drive head, the physical head that runs around the hard disk platter as you write to your drive, is doing a lot more movement as it tries to fill up all these disconnected holes with the five gigs you're putting in.
If, in both of these cases regardless of the amount of free space, your drive is thoroughly defragmented (so that all of the free space has been collected up and lives together, basically as one big empty block of free space) then I would expect the copy to take roughly the same amount of time: regardless of how much free space there is.
I think that there is a coincidental correlation between the amount of free space and the amount of fragmentation that is probably apparent on the drive.
And as the science guys like to tell us, "Correlation is not causation."
It's not necessarily the free space that means you are being slower; it's that underlying defragmented nature of the hard drive that could be slowing you down.
So what I encourage you to do is repeat your test; but this time, defrag the drives first to make sure that they're as defragmented as possible. I'm guessing you're going to find that the copy operation will be pretty consistently the same amount of time, regardless of how much free space is on the drive.
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