200GB of lost space! Difference in file size between finder and disk utility

I have installed yosemite and I am trying to have a backup using time machine. Time machine gives an error that the external drive size is not sufficient. And here begins the adventure...

I have an iMac which has hard disk capacity 1 TB (changed recently due to crash). Finder shows used space 486GB on Macintosh icon. This is more than 200GB more than my files in total (this is based first on adding the file size in finder of the individual folders within Macintosh, and then I have installed GrandPerspective and Omnidisksweeper, all lead to something more than 200GB used space). I have run in recovery mode, nothing is wrong with the disk. Any ideas? Thank you very much in advance.