SMART disk errors or NO SMART disk errors?

@mdgm: Are you sure USB-connected drives can generate SMART alerts?

I have an NV+ with 4x2Tb WD drives (xRAID) and attached 1.5 and 2TB USB drives I use for backups. I have replaced a number of drives in the enclosure over time. More recently Disk 1 has been replaced twice. The original WD Caviar Green drive generated SMART alerts @4am daily:
"Disk 1 did not pass SMART self-assessment...."

I purchased a new Caviar Black, hot-swapped Disk 1 and RMA'ed the defective drive. The new Disk 1 generated alerts from day 1. The replacement RMA'ed drive arrived so I hot-swapped again and had the Caviar Black replaced. The replacement Caviar Green - Disk 1 - has been generating the same alerts from day 1 but looks clean via Frontview:
Model: WDC WD20EARS-00MVWB0
Serial: WD-WCAZA7142785
Firmware: 51.0AB51
SMART Attribute
Raw Read Error Rate 0
Spin Up Time 0
Start Stop Count 4
Reallocated Sector Count 0
Seek Error Rate 0
Power On Hours 138
Spin Retry Count 0
Calibration Retry Count 0
Power Cycle Count 4
Power-Off Retract Count 3
Load Cycle Count 9354
Temperature Celsius 29
Reallocated Event Count 0
Current Pending Sector 0
Offline Uncorrectable 0
UDMA CRC Error Count 0
Multi Zone Error Rate 0
ATA Error Count 0

Extended Attribute
Hot-add events 0
Hot-remove events 0
Lp stat events 39
Power glitches 0
Hard disk resets 0
Retries 0
Repaired sectors 0

I checked /frontview/bin/check_smart_errors on the Readynas via ssh. This is the script that runs @4am daily and it appears to check hd's only not sd's (the USB drives are dev/sda1 and sdb1).

Am I missing something or just having a bad run with hard drives?

Appreciate any answers you can provide as this has been frustrating me for a while now.