Q: Available hard drive capacity is significantly lower than I know it should be?

I have a MacBook pro running OSX Yosemite (10.10) with 250GB hard drive, my available disk space is less than 7GB yet by my calculations I should have over 100GB of free space.

My iPhoto library is only 6GB, I have a dropbox account totalling 70GB, music is 5GB and Documents of 30GB, App's of 30GB.

Strangely when I 'get info' the size of my iPhoto library shows at 6GB but when I review this through 'about my mac' it shows at 70GB? This view also shows 'Other' as 151GB, I have no idea how his can be so high.

I have tried using disk utility to verify and repair my hard drive, the verification works but the repair doesn't and returns a message telling me to back-up and reformat my drive, problem is when i try to back-up to Time Machine this fails too despite there being ample space on my Time Machine to perform the back-up.

Any advice as to what I should do would be appreciated.