Im going to assume your drives are USB...

Most USB speed issues start at the bios level. If USB is set to legacy, slow, or 1.1 compatability mode in the bios, max transmission speed is capped by the bios. Windows will show 18 to 35 meg a second when you first start to copy, but soon realize it is no where near that and cut the computed speed back.

Usb controller types can further compound this. Most controllers are half duplex when more than one device is on the bus. Some don't support high speed at all.

For instance, USB1.1 is 12 meg a sec in fast mode, 1.5 meg a sec in slow mode. Sound familiar?

If your motherboard has seperate usb controllers, plugging the drives into different ones can help as well. But not always...

Memory allocation for the controller(s) and cpu speed also add to this confusion. If all of them are configured to use the same memory blocks, your stuck at slow speed as soon as two devices are used at once. Doesn't matter how many controllers you have then.

Then there is the power issue. If your external drives are just powered by the USB cable, the controller in the drive may default to slow speed right off the bat.

Not all cables are created equal either, there are ones that only work right on 1.1.

There were a lot of Intel chipset drivers that were incorrectly configured as well. USB 3 ports should be routed to the xHCI ports, not the ECHI ones. Updating the motherboard drivers to the newest version will usually fix this with the acception of some Win8+ drivers for oddball chipsets.

The best solution to all of this is installing an eSATA card and getting eSATA cables and enclosures for the drives. This wont work if the drives are IDE, you will need new drives. If that is the case, get a 6gig a sec card and drives. eSATA enclosures usually do eSATA and usb, so they will be backwards compatable to PC/notbooks without eSATA.

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