SSD hard drive free space question

I have an early 2011 15" MacBook pro. I recently removed the stock hard drive and out in a Crucial m4 256gb. After the transfers from my stock drive I had 178gb of free space. Since then I have removed the optical drive and added a 750gb platter hard drive using OWC Data Doubler. All went well. So, my problem is this. I moved my iPhoto library about 20 gb from the ssd to the platter drive that I put in with the data doubler. I then deleted the iPhoto library from my ssd and emptied the trash. The finder window says I now have 198gb of free space on the ssd which makes perfect sense to me. However, the program What Size and the disk utility in the utilities folder on my Mac say I have 178gb free. In using What Size and totaling the amount of space used it comes to a number when subtracted from the 256gb on the ssd leaves 198 gb free which seems right but disk utility still says 178 gb. I cannot locate the extra 20gb in the ssd. Can anyone help me with this? Thanks.

Thank you and thank you again. That was it exactly!!!!!!

Weaselboy said:

Are you using Time Machine? If so, that is likely the issue. Time Machine on a portable Mac creates a MobileBackups volume that saves Time Machine backups to the local drive when you are not connected to your backup drive. This volume will keep growing until you need the space, then Time Machine will start to delete items from the MobileBackups volume.

On my MacBook Pro the Finder shows 68.17GB free and Disk Util shows 62.64 free. The difference is exactly the 5.5GB shown as used by MobileBackups in About this Mac/More Info/Storage form the Apple icon in the menu.

If you turn Time Machine off then immediately back on it will delete the items in the MobileBackups volume and reclaim your space. You can just leave it though if you want, as I mentioned Time Machine will auto delete items from there if the drive starts getting full.

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