I have three WD Sata disks (Western Digital Caviar Green (Adv. Format) family) all with the same model no: WDC WD20EARS-00MVWB0, but different Revision No. All drives have smart support and two of the disks are smart enabled.

When enebling the third I get the following:

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
llist@LeosLinux:~$ sudo smartctl --smart=on --saveauto=on  -a /dev/sdd
[sudo] password for llist:
smartctl 5.40 2010-07-12 r3124 [x86_64-unknown-linux-gnu] (local build)
Copyright (C) 2002-10 by Bruce Allen, http://smartmontools.sourceforge.net

=== START OF INFORMATION SECTION ===
Model Family:     Western Digital Caviar Green (Adv. Format) family
Device Model:     WDC WD20EARS-00MVWB0
Serial Number:    WD-WCAZA4779401
Firmware Version: 0953
User Capacity:    2,000,398,934,016 bytes
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 Jun 28 11:39:10 2011 EST
SMART support is: Available - device has SMART capability.
SMART support is: Disabled

=== START OF ENABLE/DISABLE COMMANDS SECTION ===
SMART Enabled.
SMART Attribute Autosave Enabled.

=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED

General SMART Values:
Offline data collection status:  (0x00) Offline data collection activity
was never started.
Auto Offline Data Collection: Disabled.
Total time to complete Offline
data collection: (   0) seconds.
Offline data collection
capabilities: (0x00) Offline data collection not supported.
SMART capabilities:            (0x0000) Automatic saving of SMART data is not implemented.
Error logging capability:        (0x00) Error logging NOT supported.
No General Purpose Logging support.

SMART Attributes Data Structure revision number: 5
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
194 Temperature_Celsius     0x0022   044   050   000    Old_age   Always       -       44 (0 21 0 0)

Warning: device does not support Error Logging
Error SMART Error Log Read failed: Input/output error
Smartctl: SMART Error Log Read Failed
Warning: device does not support Self Test Logging
Error SMART Error Self-Test Log Read failed: Input/output error
Smartctl: SMART Self Test Log Read Failed
Device does not support Selective Self Tests/Logging

++++++++++++++++++++++++++++++++++++++

The only difference I can see between the drives that can be enabled and the one that can't, is the Firmware revision. Have contacted WD without any success.