WD2002FAEX intermittently missing

“…  in Raid 1 configuration. Randomly one of the drives will disappear …”

that’s what happens when using WD Desktop disks in RAID configurations

WD desktop disks aren’t supported for RAID
and according to the following WDC help page
using them in RAID voids the warranty

http://wdc.custhelp.com/app/answers/detail/a_id/1397/~/difference-between-desktop-edition-and-raid-%28enterprise%29-edition-drives