I successfully upgraded to macOS Catalina 10.15.7, but since then I could not download the latest version iTunes, nor c

Hello Felogene,

Thank you for using Apple Support Communities. Based on your post, you'd like to know how to get your Mac to see an external drive that you had previously created to backup files. We'll do our best to assist.

It sounds like when you connect the external drive, it's not showing up in the Finder. Check in the Finder under Finder menu in the menu bar > Preferences > Sidebar to make sure that 'External Disks' are enabled under Locations. You can also select others here to set your preferences, like 'Hard Disks' or 'iCloud Storage' for example.

With that said, if you were using Time Machine to backup files, then you can use the following to Restore your Mac from a backup

If that doesn't solve the issue, please reply back with the following:

• What format did you use when you set up the external drive?
• What happens when you connect the external to the Mac itself?
• Does it show up in System Information? You can find System Information in the Utilities folder from Applications. There you'll locate your port to see if the device shows up. Check this out for more details on System Information: About System Information on your Mac

Hoping this helps out, take care!