WD15EARS - 00S8B1 - slow compared to others the same type

Hi Everyone

I bought 3 of these drives and 2 of them work fine and I have had no issues with them in Linux or Windows 7 64 bit.  They are fast enough and things seem OK.  The 3rd one however makes both Linux and Windows run really slowly.  It takes more than 10 times the amount of time to format the drive and I am currently doing a 1 pass wipe on the drives to test this, at this stage the 3rd drive *the slow one* has taken 8 times as long as one of the other ones and is still going…

I have tested with the diagnostics and it say all is OK.  Has anyone got any ideas what is wrong or how to fix.  The drive is 4k aligned already - 1st thing I checked.