Ok I think this is some valuable information that needs to be out there.
As this is what I encountered with my machine.
I don't know if all machines will do the same but this is what happened for me.
Just my 2 cents worth..

A while ago there was a discussion going as the weather or not one had to steady safely remove your flash drives and external storage drives every time you want to power down/remove them.

The work around was this so we wouldn't need to 'safely remove':
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But I found out you don't want to do that to E-SATA drives as this is the result, USB1 transfer rate:
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I went and re-enabled the 'safely remove' feature for the E-SATA drives only. As it seems USB isn't affected this way:
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And we are back to good transfer rates once more:
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And yes the E-SATA dives are self-powered.

Just my 2 cents.