Different versions of WD2001FASS (2 TB Caviar Black )?

I bought a QNAP TS419P+ a few years ago.  I filled the 4 slots with WD 2 TB Caviar Black Drives.

This morning I received an error that one of the drives has failed so I called to have it replaced.  While retrieving the S/N, I noticed that all of the drives are not the same exact model.

Drive 1     WD2001FASS-00U0B0

Drive 2     WD2001FASS-00U0B0

Drive 3     WD2001FASS-00U0B0

Drive 4     WD2001FASS-00W2B0

Drive 1 failed and will be replaced.

Does anyone know the difference between WD2001FASS-00U0B0 and WD2001FASS-00W2B0???


See http://community.wd.com/t5/Desktop-Mobile-Drives/Product-number-help/td-p/270798