Thanks for your reply.
2nd drive was added less than about 7 months ago, about a month after the first drive replaced an older drive.
I have tried the reinstall controller trick, and DMA is set to UDMA 5 for both (it was set to PIO for my DVD, and reinstall of its controller has reset the DMA to UDMA 2). Cables to both drives are identical, and just as old as the drives, but I can't rule out a bad cable. MOBO has 2 main SATA connection points, right next to each other and easily accessible. I can see six hard drive positions in the bios, but I'll have to open the box to find other connection points.
I'm curious if the Speccy reading of 3.0 gb/s (= SATA/300 doesn't it?) for both drives is correct, and the Defraggler health reading of SATA/600 (=6.0 Gb/s, yes?) for one drive is incorrect.
Part of the question here is whether I have a drive working slower than possible, or if one or the other, Defraggler/Speccy, is wrong in its assessment of one of these drives.