My Book Hard Drive and update to Catalina

Hi

I have a Western Digital Hard Drive - My Book and

For various reasons I had to upgrade from El Capitan to Catalina yesterday.

I made a back up using My Book and the time machine facility facility before upgrading.

I now want to access the back up which includes lots of important files and I find that I cannot unlock the drive …similar to someone’s post on this forum.

How can I ensure that I can retrieve my back up files from My Book hard drive when I cannot unlock it.

Do I have to download a new WD security app etc ?

Please clarify.

Many thanks

James