ad0 READ_DMA TIMEOUT errors on install of 7.0-RELEASE

[Yani Karydis]

Just to add my 5 cents to this topic: I had similar READ_DMA_TIMEOUT 
problems with a Promise Ultra100 controller with 2 PATA disks that 
caused various problems including memory corruption under heavy use. As 
soon as I replaced the controller with a cheap VIA 6421 controller all 
problems went away. One strange thing was that when only 1 PATA disk is 
attached there are no (obvious) problems. The problems start to appear 
only when more disks are attached to the controller.