do i have a bad disk ?

[Chris Moore]

Code:

SMART Attributes Data Structure revision number: 16
 Vendor Specific SMART Attributes with Thresholds:
 ID# ATTRIBUTE_NAME		  FLAGS	VALUE WORST THRESH FAIL RAW_VALUE
   1 Raw_Read_Error_Rate	 POSR-K   200   200   051	-	8

This "Raw_Read_Error_Rate" on Western Digital drives is normally zero unless there is a problem with the drive.
I have a feeling that the drive is having trouble and should be replaced.
Since you are looking to replace all the drives, one at a time, you should start with this one.