I like to transfer certain files from the older one (6 months old) to the new ( 1 day old). If I plug in both USB only one of them shows up.
It’s a Windows issue. A quick method is to go into Disk Management and change the drive letter on one and make sure they are both online. This post has a permanent fix http://community.wdc.com/t5/My-Passport-for-PC/Copy-files-from-one-drive-to-another/m-p/277504/highlight/false#M7587 see post qq.