Cannot see TWO drives simultaneously (4TB USB 3.0 My Book Essential); only one at a time

WD External Drives External Drives for PC
3

It sounds like a Windows Disk Signature problem. See if this helps fix the problem   http://community.wdc.com/t5/My-Passport-for-PC/Copy-files-from-one-drive-to-another/m-p/277504/highlight/false#M7587 see post 11.

Joe