I can't enable SMART on this drive.
-s option, suggested by the output (see below) doesn't help.
$sudo /usr/local/sbin/smartctl -a /dev/ad4
smartctl 5.42 2011-10-20 r3458 [FreeBSD 8.2-STABLE amd64] (local build)
Copyright (C) 2002-11 by Bruce Allen,
=== START OF INFORMATION SECTION ===
Model Family: Western Digital Caviar Green (Adv. Format)
Device Model: WDC WD20EARS-00MVWB0
Serial Number: WD-WCAZA0719722
Firmware Version: 0909
User Capacity: 2,000,398,934,016 bytes [2.00 TB]
Sector Size: 512 bytes logical/physical
Device is: In smartctl database [for details use: -P show]
ATA Version is: 7
ATA Standard is: Exact ATA specification draft version not indicated
Local Time is: Tue Dec 6 00:06:45 2011 PST
SMART support is: Available - device has SMART capability.
SMART support is: Disabled
SMART Disabled. Use option -s with argument 'on' to enable it.