Welcome to Tom's Hardware, mate!
Since the 1 TB external drive does show in Device Manager, you should also check
Disk management in Windows and see how the HDD pops up there. Posting some screenshots from there would be really helpful, too.
If you want to re-format the external HDD, you can also do it from there by deleting the volumes on the drive and then create a new one. Here's a detailed tutorial that could help you as well -
How to partition & format your drive. Make sure you select the right drive, otherwise you might format the wrong HDD.
If you still face issues with the detection of the external in Disk Management, you should try plugging it using a different USB cable and to a different port on your computer. If you have access to another PC/laptop running Windows OS, you might as well try to format the hard drive from there.
Keep me posted if you have more questions! Hope this helps you!
SuperSoph_WD