MacBook Pro hard drive full, files missing

Nick_Macartney wrote:

Macbook pro os 26.1 I was copying from one external drive to another and suddnely noticed my hard drive almost full. About 300 GB have gone onto it. I cannot find anywhere the files that have suddenly moved onto the hard disk. I searched all files.

All Volumes within a  Container share space without penalty—giving and taking as needed:

Trash

Time Machine local backups

Virtual memory

cache memory

tmp files

etc...

reboot the machine, do a Safeboot all effect what is presented in your "System Data"

What Apple used to label "Other" is now labeled "System Data".

Empty your Trash — Delete files and folders on Mac - Apple Support

Delete files and folders on Mac - Apple Support

How to free up storage space on your Mac - Apple Support 

Free up storage space on your Mac - Apple Support

See used and available storage space on your Mac

See used and available storage space on your Mac - Apple Support

Try something like OmniDiskSweeper for a GUI to get a good look at itemized file size and location:

OmniDiskSweeper http://www.omnigroup.com/more