Why do HDDs do this?

Wolther · June 28, 2016

I tested this with a big folder of files. (ish) it's about 26 GBs and while watching the progress bar, sometimes transferring pauses and goes down to 0MB/s and then quickly jumps back to 50 MB/s - 10 MB/s a second. I know the fluctuation of transfer speed is due speed depending on each size of the file it's working on. But why does it sometimes outright stop for about 10-20 seconds and start up again?

juretrn · June 28, 2016

Maybe another program (with a higher prority) was accessing files on the drive?

mariushm · June 29, 2016

Antivirus software running in the background can cause such delays.

The operating system also caches often accessed files in memory. It's possible from time to time for the operating system to completely fill the unused RAM in your computer with the accessed files during the copy process, and then the operating system spends a bit of time removing some of the cached files to make room for new content.

If your hard drive is very fragmented or you have a lot of files, the information about the folder structure and where files are located can often be stored all over the hard drive in several places, so from time to time, the operating system may have to search in various places on the hard drive for the information about WHERE the following files to be copied is actually located.

Wolther · June 29, 2016

4 hours ago, mariushm said:

Antivirus software running in the background can cause such delays.

The operating system also caches often accessed files in memory. It's possible from time to time for the operating system to completely fill the unused RAM in your computer with the accessed files during the copy process, and then the operating system spends a bit of time removing some of the cached files to make room for new content.

If your hard drive is very fragmented or you have a lot of files, the information about the folder structure and where files are located can often be stored all over the hard drive in several places, so from time to time, the operating system may have to search in various places on the hard drive for the information about WHERE the following files to be copied is actually located.

Thank you It made sense with a lot of what you described.

scottyseng · June 29, 2016

10 hours ago, Wolther said:

I tested this with a big folder of files. (ish) it's about 26 GBs and while watching the progress bar, sometimes transferring pauses and goes down to 0MB/s and then quickly jumps back to 50 MB/s - 10 MB/s a second. I know the fluctuation of transfer speed is due speed depending on each size of the file it's working on. But why does it sometimes outright stop for about 10-20 seconds and start up again?

The other thing I can think of that wasn't mentioned is if the drive is failing and it ran into a corrupted sector / had to move the data to a sector that is good. That orr the drive ran into a error is trying to fix it before moving on.

Usually though, it's either you're moving a lot of small files, corruption or hardware failure, or something else is taking priority.

mariushm · June 29, 2016

Yeah, I forgot to mention that.

I had a hard drive that had some bad sectors in an area, every time it read files from that area of the drive it would slow down. Also, while playing some movies, I'd get from time to time corrupted frames due to data incorrectly read from those sectors.

I've temporarily fixed the issue by making several partitions, and one of those partitions covered that bad area of the drive and didn't get a drive letter.

I've fixed the drive completely by moving all the data I cared for from the drive to another and then used Western Digital's Diagnostics Tool (it also works with drives that are not made by WD) , it has an option called "Write Zeroes" which forces writes on each sector of the drive, making the hard drive re-evaluate bad sectors and completely marking them as bad if needed, so the drive will no longer slow down in that area. Takes a few hours for a 2 TB drive but worth it.

Of course, the drive still has bad sectors but doesn't slow down anymore, so I only use it for movies or series I still like to view from time to time but I can find again or re-rip from the DVDs I own if the drive dies. After all, it's 2TB of disk space.