time machine backup

The best practice is to use a dedicated drive. But, you can use a drive that you have with other files as a stop-gap. Note that Time Machine prefers APFS format and will ask you to reformat the drive. So, that may be a consideration too. https://support.apple.com/en-gb/guide/mac-help/mh15139/mac

Also, as a general rule of thumb it is wise to replace HD's about every 5-6 years as they wear out. When it comes to backup it is wise to do this!!! It is also wise that you can replace the HD without replacing the whole enclosure, this is one (of many) reasons a lot of experience users use the OWC Mercury Elite Pro as changing out the HD takes 5 minutes and a simple screw driver.