While I use Mint 18.3, this is NOT a LINUX question because the problem is even worse in Windows (any version). Also I have a home-built state of the art gaming machine that is blazingly fast. Gimp and Photoshop load in about 5 seconds the first time and instantly the second time. At work it takes 30 seconds or more every time. I use a 500GB SSD for the operating system and a bank of the latest WD drives for data. So it is not my hardware.
When copying or moving files (very large 200-500 Gbytes directories) using drag and drop (in either LINUX or Windows-any version-this has been an ongoing problem) the transfer rate gradually slows down overtime. In Windows it virtually stops after a an hour or so and in LINUX it slows way down to as little as 4-10 MB per second.
Yet when I use the CLI, transfers do not always slow down (depends which application is used). In LINUX, using rsync to copy a 300GB Home directory onto another drive only takes about an hour. But if I drag and drop, it slows down so much that I just give up and use rsync even though I hate using the CLI. In researching this I found people complaining about this problem using DD copying drives but I have never used DD so I don’t know. In windows its the same. Some Windows CLI commands will move and copy files very fast without a slowdown.
The question comes up surprising little when googling it. But when it does it is always related to a specific situation and the answers try to address aspects of that situation rather than address the underlying endemic problem unrelated to any specific situation. Its always true irrespective of the situation. I have noticed this for many years and many operating system versions (except not when using DOS, CPM, VAX-11, and IBM which were CLI only) copying all sorts of things to all sorts of storage devices using many different computers. Given this, it must be a software and not a hardware problem, most likely the way the buffer is being utilized??
The usual answer is that once the memory buffer is full, the transfer continues at the native-unbuffered write speed. While that may be a contributing factor I do not believe that the native write speed is less than 10 MB per second especially on an SSD! And it doesn’t explain why there is only a small slowdown using the CLI and a huge slowdown using the GUI drag and drop.
Are there any hard drive experts out there who know about this? Hopefully some WD engineers will address this question.