Testing many SATA lap drives, major mfgrs, several capacities, I've got one or more sharp dips in the speed graph (down to almost zero). All disks pass the Windows 7 tests and Acronis Disk Manager SMART tests. All seems to work ok. The dip location is erratic, so I don't think it is showing a bad place on the platter. The sequential read progress bar pauses at about the same place as the dip appears. What causes this? Is the disk bad? Is it ok to use?