Why does USB transfer start at like 400MB/s then decrease?

[fuzzybabybunny]

I'm transferring in Windows 7 a movie from my SSD to an external HDD. At first it transfers at like 400MB/s but then slows down and plateaus at 30MB/s. This is hardly abnormal, but I'm just wondering why does it start off so strong and then slow down? It's a single large file. Or is the Windows Explorer transfer speed it displays just inaccurate?

 

[nickbits]

Slows down until the error rate is minimal.

Actually I have no idea but that's my guess.

 

[razel]

It's probably caching to RAM or incorrectly estimating. I noticed a very similar thing but worse. Whenever I do larger than RAM copys to USB externals to starts off great at 30-35MB, until it reaches to roughly the same amount of RAM in the system then it just drops to 16MB. Not even using TeraCopy helps allieviate this.

Those were all USB 2.0 2005-2008 era laptops/desktops. Then one day one of the laptops was a newer Sandy Bridge laptop. Low and behold it was consistent at 35MB. Could it be just the USB ports or Windows itself? I don't know.

 

[zuffy]

RAM caching.

 

[greenhawk]

RAM caching.

+1, then the sustained speed is the actual USB speed.

 

[Shmee]

I assume this is USB 2 and not 3?

 

[Nothinman]

I'm transferring in Windows 7 a movie from my SSD to an external HDD. At first it transfers at like 400MB/s but then slows down and plateaus at 30MB/s. This is hardly abnormal, but I'm just wondering why does it start off so strong and then slow down? It's a single large file. Or is the Windows Explorer transfer speed it displays just inaccurate?

Write caching. Windows will only buffer so much before flushing it to disk so to explorer the first bit seems to fly and then slows down as the buffer has to be written to disk. That's also why you can lose files if you pull it out without ejecting first, although Windows tries to minimize that now by mounting removable media synchronous.

 

[Charlie98]

Write caching. Windows will only buffer so much before flushing it to disk so to explorer the first bit seems to fly and then slows down as the buffer has to be written to disk. That's also why you can lose files if you pull it out without ejecting first, although Windows tries to minimize that now by mounting removable media synchronous.

That explains a lot... the writing to USB part... Makes sense. :awe: