Purgeable memory

Purgeable disk memory is disk memory that has files stored in it. The most frequent creator of these is "snapshot Backups" created when your regular backup drive is not available.

Purgeable means that they will be removed instantly if any other Application asks for file space that is not available otherwise.

Time Machine makes automatic incremental Backups about hourly. It does so at such low priority, it barely disturbs what you are doing. If you do not regularly connect the Backup Drive, turning off these snapshots will leave you with no backups for the time you do not have the external drive connected. These files are not clogging up your drive -- as I said before, their space will be yielded instantly for new files if no other space is available, so these files are not "in your way". I do not recommend removing this safety net, just to have bigger numbers under "free space" on your drive.

If you are adamant about doing this, a quick search will bring up a Terminal procedure to turn off the local snapshot backups feature. Cut and paste the command from the article into Terminal.

if you have time machine on you should first turn it off and then restart your mac then you will see the biggest different in purgeable ok then you can turn on your time machine my was up to 400.00gb and then once i did what i just told you it when down to 2.37gb ok hopefully this solve your problem you just have to do over and over that i s the only thing working for me ok

This has been the most helpful comment I have come across in 2 days of searching for a similar problem. It is concise and plainly explains the issue - a pity Apple do not provide similar clarity on their 'support' pages.

Other users have mentioned altering Time Machine settings and re-starting etc. I had tried this and found my purge section went up from 56G to 68G.

Following an update to Sierra, and a couple of stumbles along the way, my laptop had shown approx 145G used, 117G purge, 56G free, from the storage feature under 'About this Mac' under the Apple Logo; if I used Finder I was supposed to have 145G used, 173 free.

However after reading Grant's advise I ran a full back-up and found my drive looked more like it should; 145 used, 173 free.