My WD Elements 2TB External HDD is losing it's transfer speed from day by day, why is that?

[BladePocok]

I've recently (like a month ago) bought a brand new Western Digital Elements 2TB version . In the first few days (week) it was perfectly fine, with a speed of around 95 MB/s. Started to fill it up with movies, series and so on and by doing that, it slowly started to slow down (to 85 MB/s in 2 weeks time and nowadays it's around 75 MB/s). Out of 1.8TB, 1.24TB is used so there are around 580GB free space left. How should reverse/stop this from happening? Is it a sign of failure? Or it's a natural thing when the space is filled up that much in such short time? (as I said, bought it about a 1.5 - 1 month ago). I have Windows 10 system on a NVME SSD and from where I transfer these files is a 1TB WD Caviar Blue. Is it maybe the 1TB internal hdd is dying and due to that fact the transfer speed is decreasing too ?

 

[gasaraki]

Try defragmenting it.

 

[Solandri]

Two things going on:

• 
  ■Fragmentation. Drives can only read their full speed reading/writing large sequential files. The process of moving the read/write heads to find a new file is glacial (in computer terms). This is why HDD sequential speeds are typically faster than 100 MB/s, but they can only manage about 1 MB/s when it comes to small random files.
  
  As the drive fills up, there will be less contiguous free space. So the drive will be forced to split even large files into 2 or more pieces. The more pieces, the slower it'll be to read the file. Defragmenting the drive can help reduce this problem.
  ■Track length. Data is written onto circular platters. Since the entire platter moves at the same speed, the outside tracks move a lot faster than the inside tracks. Since the smallest piece of data which can be written is based on the size of the read/write head, you can fit more data onto the outside tracks than on the inside tracks.
  
  Consequently, sequential reads/writes on the outer tracks are faster than on the inner tracks. To take advantage of this, HDDs start by writing data to the outer, faster tracks first. HDDs typically have about a 2:1 ratio between their outermost and innermost tracks. So the sequential read speeds you'll get will vary by up to 2:1 depending on which track the data was written. If you were getting a max speed of 100 MB/s, that was probably data on the outer tracks, and you should expect a max of 50 MB/s for data written on the inner tracks.
  
  There is no way to "fix" this. It is simply how HDDs work. (Well, you could fix it by writing the same amount of data to each track, but that would make the entire drive operate at the speed of the slowest innermost track. Your drive would then transfer at only 50 MB/s and would only have 67% its current data capacity.)

 

[BladePocok]

So to defragment an (external) hdd is fine, but the overall problem won't change unless I free up some space.

 

[Solandri]

I wouldn't exactly call it a problem. Every HDD is affected by it so you can't escape it (except by switching to SSDs). But yes, you can minimize its impact by buying a much larger drive than you need and leaving lots of free space. One of the tricks people did before SSDs was called short-stroking. You make a small partition for the OS first, then create a larger partition for data. This puts all OS files on the outermost tracks where the drive is fastest (hence the name - the read/write heads only have to travel a short stroke to reach the OS files). Data whose speed isn't as important ends up occupying the inner tracks.

I probably should've mentioned the effect of fragmentation is much bigger. Whereas writing data to the inner tracks will only yield about 50% of the max speed, fragmentation can drop the speed down below 1 MB/s (which on modern drives is less than 1% of max speed). It's why when you try to copy a bunch of small files (like MP3s) from one partition to another on the same drive, it usually proceeds at less than 10 MB/s. The drive is spending most of its time waiting for the read/write heads to move back and forth, instead of reading and writing data.

Incidentally, HD Tune is one of the better HDD benchmark programs which will measure transfer speeds across the entire surface of the drive, not just with a single file. In the resulting plot, you can see how the speed (blue line) drops to about half its max as you get to the inner tracks of the drive. And the seek times (yellow dots - time to move the read/write heads and wait for data to spin under the head) increases for the inner tracks (which are furthest from the read/write head rest position. The vertical spread of the dots reflects how long the drive has to wait for the data to spin under the head. The bottom is when the data arrives just as the head does. The top is when the head just misses the data, and has to wait a full platter rotation for the data to come around again.

http://www.hdtune.com/

I'm not sure how useful a HD Tune benchmark will be on a full drive like yours. But you can run it on a new drive to get an idea of the best and worst case performance you should expect as you fill up the drive.