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Copying several large files ( > 2GB ) between two USB HDDs reveals the I/O pattern below in Process Explorer.

Write caching is disabled on the destination drive. I would have expected I/O to run at high speed until the device's internal cache filled up, at which point it would slow down for the remainder of the transfer.

Instead, we see spikes of high I/O, followed by a downward slope before it drops off entirely, only to resume a few seconds later. The HDD activity light remains on the entire time.

What is going on here? Why the complete stalls? I would presume they happen when the drive's internal cache is full, but it seems to then refuse any further transfers for a short while, then return to full speed (indicating that the cache is possibly empty).

The source drive is a Seagate 500GB SATA connected via USB, the destination drive is a 2TB WD SATA also connected via USB.

Process Explorer screenshot

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Maybe the source drive has bad sectors? – Dec 25 '11 at 7:49
up vote 0 down vote accepted

Those look like very long pauses. Without looking at Procmon, there can only be speculation.

You should use Procmon as it is impossible to tell what is going on. Task Manager can only tell you the symptoms. What we need are the kernel calls, especially the cacheman calls.

Like you said yourself, my initial guess is also when Windows makes a cache flush call at the end of a file.

Using Procmon, you can usually see the flush call. If you log events from the System process, you can see the File System driver make a FlushBuffers call.

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Will do that and update when I get a chance. – Unsigned Dec 26 '11 at 0:59

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