that is the Old "how long is a piece of string" question...
you can use usb dumb drives/stick as holder for rarely used files.
there are a load of apps on the App Store for finding and deleting duplicated, nsnf slow trmovinr lrsdy udr, or largest files. etc. and also delete and iPhone backups that may be on the main hard drive..
of be like me with over a dozen different Hard drive, which hold different Large files auguring to their usage and importance.
How big is you drive and what computer info would help.
Regards, Sandy