Using external drive for Time Machine & storage of other files

you could partition the drive into two parts then use one partition for TM, and the other to store your extra data. but if I may add my two cents, that is an extremely bad idea. if / when the drive fails, you lose everything. as affordable as external HDDs have become, there's no reason to not have two (or more) drives and dedicate each of those for those two purposes.

Are the files you are (or were) planning to store on the external drive's data only partition important files? Would this be the location of the original files? If so, how will you back them up if they are important & the only copy? Adding them to the TM backup which is stored on the same drive isn't a backup when you consider what happens to that backup if that drive fails....both the TM backup copy & the original files on the data only partition would both be gone.

Besides, just mixing a TM backup with storing extra copies of data on a data only partition just puts your TM backup as more risk with all the extra activity on that physical drive. Something is much more likely to go wrong and compromise the TM backup.

For a third point, I never recommend multiple partitions on any drive because people realize much later that one or more of those partitions is too small. Then they have a huge mess & lots of work to resolve it all the while risking their data.

Instructions are here:

Use Time Machine to back up - Apple Support

But I agree with jefferythefrog, dedicate an external drive to Time Machine backups and get another external drive if you want additional external storage.

you're welcome.