Hi Drew.
As a general rule, the optimal space in a hard disk in order to keep it fast, is 65% of the space, because, even if that occupied space would be only text in small files, there is always the chance that at some moment you had the whole disk full and pieces of those files deleted are on the "unused" half and making the start at least slower.
But to gain an optimal speed, close to the specifications of the Hard Disk maker (which are always the maximum speeds the HD can reach, when exposed to heavy duty, not the average speeds) is more complex than that, not uneasy to understand. I'll explain.
The file that makes the HD the slower is the photo, a photo is a compressed file, most formats of photos are compressed files, when a photo is at its full size, the operative system takes much time to open it, since every dot of the image it has a mathematical formula that represents its color to be put on the screen, a position, and some data put there like the name, which is something windows have to decode, and interpret in which part of the Hard Disk resides, all of this makes the opening of a photo a delay in the run of a Hard Disk.
With a compressed image in the popular format JPEG there should not be problem, and it is like this if the photo is smaller than, lets say 500 Kilobytes. You should check the system of measuring data, or units of information http://en.wikipedia.org/wiki/Kilobyte
But with only knowing for the moment how much takes you to open a photo of 100 Kb, one of 500 Kb, one of 3 Megabytes, you'll understand thorough the sensation of delaying, how important is every size of a file for your computer, your hard disk and for yourself.
When you have a large amount of photos of let's say 100 Kilobytes each one, like 1000 photos by folder, the hard disk will take time to make them to show their thumbnails, that's what i told you before when i said that windows have to check them first before you can visualize each one. That is stored in the local file that every single folder has, which is thumbs.db. It is a database of what you have in any folder.
And added that to the fact that windows stores that in another master register (which is in the system folder inside the Windows), the delay is inevitable.
People who has photos of 1 Megabyte in an amount of 100 or 1000 per folder and dozens of folders full of them, will have those waitings at the maximum. Worst even if videos like those of 100 Megabytes, 1 Gigabyte are stored in a lots in a single folder.
My long experience has taught me that you should have no more than 4 GB (Gigabytes) per folder. Why? that thumbs.db file is in part responsible for that, if you pack a lot of videos, like 100 of 100 GB in just one folder and then want to copy them into another folder, you will be overheating the hard disk because that transference takes time and you wouldn't be letting it time to sink the heat and breath, yes, Hard disk breath and equalize the atmospheric pressure through those thin foils of aluminum.
The makers usually say this is a lie, because they don't want people to open them, or pour water to them or other imprudent things like those. But without those foils with micro holes
the hard disk would explode because of the accumulated heat.
Another problem is the fragmentation, it occurs naturally when a blackout occurs while your hard disk was working, when you reset repeated times your computer, and many times, after all of that has happened and you simply start your computer.
Fragmentation happens principally when a file was erased and windows accommodates the space where that file was, and given
that a hard disk is divided in segments of exact sizes, like in a cake, it is impossible to avoid
fragmentation.
You should defragment your hard drive after you see that is way too slow. You'll see, it's not a good idea defragment every month as the old tradition established by Microsoft taught. Why don't do it so soon? When a blackout occurs, a multiple pushing of the reset button is made by your little sister, the hard disk begins to scratch, the writer arm does that.
The fragments grow with time, and they are responsible by most overheat than when you watch a movie or clone your hard disk or copy its content folder by folder into another hard disk or unto itself.
Because of this is that everyone should have a high speed fan, a large one, cooling the whole computer, not only the hard disk, those that go attached to the hard disk are a good idea.
If you are going to use mineral oil for your computer, the hard disk doesn't go sunk into it, just as the DVD recorder.
You can defragment your hard disk if you are copying constantly videos, creating, editing, but use always a strong fan to cool your hard disk.
One thing more, your hard disk is at the limit of slow hard disks, hard disk with more speed are those of 400 GB and over. Even if the highest transfer speed acknowledged by the manufacturer says 300 GB/s usually that, as i said, only happens when you are doing intensive work, and typically never reach that speed.
To reach that speed, you need an expensive motherboard, and almost all the space available.
PD: The smallest the hard disk the lesser the chance of having
more plates for recording, the more plates the bigger the
chances to let one plate to do a task while the other is
working into another, thus, we can speak about speed,
which is in reality, just an elimination of a bottleneck, not
raw speed per se.
I know i talked about windows but it is essentially the same
mechanics working with another Operative System like Mac.