When moving big files from a SSD to a HDD on my local computer, I sometimes see the first part of a big file (several GB) being moved in literally milliseconds. However, I haven't been able to make a picture of that until now:

First 30% of ~ 4GB file moved in fractions of a second

Please note that the size of the file being moved is about 4.4 GB.

Now the question is: Is Windows (10 x64) showing me correct progress updates, and if yes, how can write speeds that high be possible with an ordinary, now several years old HDD?


  • HDD in use : ST3250312CSS
  • SSD in use : Kingston SSD SV300 (seq. write / read at about 450 MB / s)
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    Windows does lots of things to make copying a file more effective. It first will cache the file into memory, it will continue to do that, while writing the file to the desination location. The SSD is extremely fast to write. The progress is only an estimate, its based on that initial cache write, and mostly accurate. – Ramhound Mar 23 '16 at 12:29

It is using the HDD cache (i.e. memory).

The SSD has high read speed and fills up the cache on the HDD. Once there is data, it starts to write - but it is not writing at the rate it is receiving.

Once the cache is full and the HDD is unable to clear it by writing fast enough, the speed drops to the more normal speed expected of a HDD.

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  • 2
    The cache on a HDD typically is in the order of 64MB. So this will not account for the speed difference for more than a 1/10th of a second. Now if you mean the general diskcache of the OS then you are right. – Hennes Mar 23 '16 at 12:38
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    I believe it's OS write cache that plays the most important role in such a high speed rather than HDD cache, especially if there's a lot of free RAM available. – Alexey Ivanov Mar 25 '16 at 11:56

If your drive is connected to a SATA port that has Hot Plug enabled, or to a eSATA port or USB port through some adapter, you may want to see if its Removal policy is Better performance:

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If so, change it to Quick removal to stop Windows from using huge amount of RAM for write caching:

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Note that despite the description, it does not disable the actual write cache in the disk. This is how it looks when the disk cache is disabled:

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You can also try disabling both and see how things go:

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Normally it is not recommended to disable the write cache in the disk though, because the performance impact can be huge and if it's an SSD, disabling the disk cache can cause impact on write amplification and hence its life span.

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