Swopped TimeMachine HDD and now can't see old HDD

I am using TimeMachine with an old HDD (MyBook, externally powered). It works fine. I bought a new WD MyPassport Ultra (USB-powered). I told TimeMachine to use the new HDD. It worked fine.

My old HDD had archives that are not part of the TimeMachine backup. When I plugged in the old HDD, I can no longer even see it on my desktop. What happened?

Background: the old HDD was password-protected. When I plug in it before, I can see the drive and when I try to open the HDD I have to type in a password. For the new HDD, I had to reformat into Mac format and then download a security software from WD to enable me to set a password for the new HDD.

Did I accidentally overwrite the old security software that allowed me to see and password-open the old HDD?

If so, how can I bring back or re-download the software that will allow me to see the old HDD and present me the window in which to type my password to unlock the old HDD?

Perhaps I should have allowed TimeMachine to backup to both old and new drives but I chose the option to backup only to the new HDD.

Thanks very much!  I need the solution to access my archives in the old HDD.

Hello, I don’t have a lot of experience with Time machine I recommend you to check the Apple forums, or maybe some of the other users can participate and give you some suggestions. 

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