IDE screwup
When I replaced the faulty drive yesterday, I discovered that I had screwed up my IDE configuration: both mirrors were on the same IDE bus, one master & one slave. This meant that it was almost always saturated. I fixed it, and now everything is fast, blazing fast.
For those who don’’t know, an IDE bus can handle one or two drives. If two, the one at the end of the bus must be configured as the master and the one in the middle as the slave. Nowadays many drives have a ‘Cable Select’ option which is supposed to negotiate the master-slave relationship, but that’s untrustworthy. If you’re mirroring two drives, make sure that they are on separate buses. Also, the master typically gets better performance than the slave — make rarely-used drives like CD drives the slaves if possible.