You are smart to ask. When a hard drive gets too full it can stop functioning and no-one wants that. Consider this, it does no good to have your Time Machine backups on your hard drive. If it fails, ALL is lost. It is always best to have an external drive which is used for the Time Machine backups. These days they are very economical, compact, easy to install with the USB cord provided and most do not require external power. You may have to reformat it, though. Once you Set Time Machine to use the external drive you can delete the old Time Machine files from your hard drive to clear the needed space. Easily done.