Hi, this may be the wrong place to ask this (how many times do people say this, and yet...) Anyway: why is it that as I'm imaging a drive with dd or dd_rescue, it starts off at 30MBps, but then as time goes on- even if the computer is not being used, or making accesses to the drive aside from the image process, it will eventually drop to 18MBps, 15MBps, and then eventually steady at 1,030KBps. Why is this? I at first thought it was because the data was being cached in RAM, and eventually the buffers filled up. But if I kill the task, and then restart it, starting from where I left off, the speed starts at around 1MBps. I looked it up, and I cannot find anything on it. Any ideas?

If the drive spins at constant speed and the blocks start at the outer edge (sane) then some slowing down is expected because the data density is higher at the outer edge than the inner as explained here but the expected slow down is about 50% not the 3% you report.

Another possibility is that the slow portion of the drive is caused by bad blocks. Drives automatically map bad blocks to a spare area. When reading this requires the head to move to the spare area, slowing down the transfer.

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I agree, and think that this is the likely cause of a major slowdown.