Windows 7 won't recognize a second MyPassport drive

I am using a 1TB MyPassport drive to store my photo library, and I have just purchased a second 1TB MyPassport drive to back up the first one. Windows 7 (64-bit) recognizes each drive, if it is plugged in by itself. However, if I plug in both MyPassport drives (which I will need to do in order to perform a backup), only the first drive plugged in is recognized by Windows. What is really odd is that both drives show up on the ‘Safely Remove Hardware’ list. I have installed the 64-bit SES drivers.

Has anyone else had this problem, and can you suggest troubleshooting or resolution steps? Thanks for your help.

It sounds like a Windows signature collision problem. This thread explains what to do 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