What is the preferred format for Time Machine drive?

Welcome to Apple Support Communities.

If you are using macOS Big Sur or later, use the APFS format. If you are using macOS Catalina or earlier, use Mac OS Extended (Journaled).

Also, what backup drive did you purchase?

Jack

I would suggest that you purchase a premium, desktop-class drive for Time Machine and return your WD drive. I have seen the cheaper portable drives fail quickly with constant use, such as backing up with Time Machine.

A good brand for drives is the premium brand of Western Digital, SanDisk Professional. A good choice is a SanDisk Professional G-DRIVE - Western Digital.

Jack

If your computer is only a few years old, APFS.

In fact, if TM does not like the format, it will erase and reformat only.