Write-cache on ATA devices is controlled by sysctl, and can be set at boot via /etc/sysctl.conf, or at runtime via the sysctl command. The sysctl you want to use is hw.ata.wc. Setting it to 1 enables the write-cache for all ATA disks, and setting it to 0 disables the write-cache for all ATA disks.
Note: disabling the cache will seriously hamper performance for ATA disks.