Earlier I inserted a SATA drive into a simple one drive backplane I have. After a "detect hardware changes" the drive appeared, and worked correctly. Later, I removed it.
Since that time, my primary SATA drive, a Western Digital Caviar Black 640GB, which was never disconnected, is extremely slow, on the order of 4.0MB/second maximum transfer rate.
The first thing I considered is that the drive might be failing, but it passes the DataLifeguard Quick Test and Windows chkdsk. While this cannot rule out a hardware problem, it indicates against it.
Can my insertion and removal of another SATA drive have reset the connection into some very slow compatibility mode? How do I fix it? If that is not the problem, what else could cause this?