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Would specifying the /MT option in robocopy give me better or worse performance, when copying a large amount of smaller files (~10-150kB) from an USB attached drive to an internal SATA drive? Should I go all crazy and specify /MT:128?

Both drives are regular 5400k spinning hard disks.

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You say USB attached drive, but you didn't mention the media type. Is that flash, or a hard disk? Why not just try a few options and see what happens? – Zoredache Mar 14 '11 at 23:50
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It would take less time to test than it does to ask the question and then wait for answers. – John Gardeniers Mar 15 '11 at 2:23
Both sides are regular "spinning" hard disks – Tobias Hertkorn Mar 16 '11 at 9:36
@John I know. But I was working on setting up the server, etc and knew that I will get to the copying in a couple of hours. So it seemed to be the reasonable thing to do, to quickly ask the question, keep working and hope for a quick answer. Sorry... – Tobias Hertkorn Mar 16 '11 at 9:44
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In this case, I would predict worse. You want to minimize contention for IO if the limiting factor is seek/local access times. Since you are not traversing a network, that will probably be the case. (Unless you have a SSD as the source.)

Of course, test it out, but I would be surprised if you got noticeably better performance. Let us know what you find.

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Unfortunatelly it totally depends. After a little testing I found out that the most important differentiator is the Operating System used to start the copy.

On XP/2003 the throughput went from okayish to horrible, at least on the setup I tested.

On a 2008 R2 the throughput went from okay to way faster, when I used the /MT:128. It seems as if the OS is smarter in planning disk access...

For me the copying was a one-time thing, so I won't investiagate any further. But the findings strongly suggest that there unfortunatelly is no general answer to that question.

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