I thought I would post this as I spent a lot of time finding the solution and came across a lot of threads for the same problem, so hope it helps some people out. I confess that this is not my work but a few cobbled together solutions from others that worked for me.
If your HD or (as in my case SSD) is reporting via Finder or Omnidisk sweeper or similar App that the used space is greater than the files that you can account for (ie my 250GB SSD was showing in Finder as having 134Gb of used space but the sum of all the files reported by Omni disc sweeper, Finder and Disk inventory X only came to 64GB with just over 50Gb of free space: hence 70GB that I could not account for or find. It seems it is usually due to hidden files and particularly those related to mobile backups from Time machine. However, knowing this and finding the pesky hidden files took me a lot of googling and a fair amount of time, repairing disk permissions and disks had no effect whatsoever, so here is the solution if your boot drive seems to have used space that you cannot accont for, or "lost space":
First of all to find the missing data:
It is usually in hidden Files. To view, you need to run OmniDisk Sweeper or similar from the root directory (very important as it will not show all the hidden files or other users files if run as I guess most people - myself included - run it, that is from their user directory) You also need to be logged in as an Administrator. Type the following into Terminal:
You will then be required to enter your password.
I used OmniDisk Sweeper (free App) with the above command to run it from the root directory, should look something like this, showing all files, hidden or otherwise and file sizes. (Must be run as User with Admin privileges).
In my example there are 68.3 GB taken up by the .MobileBackups file that wont be shown by Omnidisk sweeper or similar App if run from users directory. The .MobileBackups file is a Time machine back up file that will usually be deleted once Time Machine has backed up to an external file. However in my case, I didn't have enough space for the time machine back up so disabled it but then ended up with a hidden file called MobileBackups.Trash that took me ages to find using the above process and then delete (as I will explain in a bit).
See following article to explain further about this step:
To delete the mobilebackups.trash file I used this command in Terminal. Obviously it will need amending to delete any other files you want rid of.
sudo mv /.MobileBackups.trash /Trashme
From this link:
This will create folder in top directory of hard disk (double click on ssd or hd image to open into this directory) called “trash me” That contains the mobile backup.trash files to be deleted.
Other useful commands in Terminal
Show hidden files:
defaults write com.apple.finder AppleShowAllFiles TRUE
Stop showing hidden files:
defaults write com.apple.finder AppleShowAllFiles FALSE
I hope this is helpful.