SATA controller card -- WD4000AAKS OK, WD10EADS bad

Windows XP Home SP3 runs very, very sluggishly and reports I/O errors with certain hard drives connected to a Silicon Image SiL3512 SATA PCI controller card (two samples), but only when another HD is connected to the motherboard’s built-in controller.  If both HDs are connected to the Silicon Image card, everything is fine.   Other controller cards, based on VIA or Promise chips, work fine in all of these configurations and with all HDs.

The problem HDs are WD10EADS and Samsung F1.  HDs that work fine include the WD4000AAKS, Hitachi 7K1000.C, and Seagate 7200.12.

I slowed the drives to 1.5Gbps and tried the Silicon Image card in RAID and non-RAID configurations, with several BIOSes and drivers.  Also with the WD drives I tried many configurations for IDLE3 and TLER.  Nothing mattered.

The motherboards are based on Intel G41 and Nvidia GeForce6100 and Nforce6 chipsets.  Configuring the motherboard for AHCI or IDE mode or turning off its RAID made no difference.

What’s going on here?