freeing up hard disk space

I have used Daisy Disk to find out how hard disk space is being used. Under my name as a user, there are 52gB used in "Mobile Documents". How do I find these, and what IS a mobile document? When listing the hard drive contents, my user name has about 76GB used, but the folders under my name only add up to less than 11. The difference of about 65 does not show up. At the moment the spare space is only15, having eliminated all the allowable files.and I need 28 to instal Windows 7 or other, so that I can instal a 250GB SSD which only comes with Windows software. Please help

The best way to make space on a drive is to get (it's free) the appropriate version of Onyx for your system and run it in Clean mode with all the default settings. (www.titanium.free.fr)

But I'm curious, the SSD should be usable with OS X no matter what kind of installation software it may have come with. OS X and Disk Utility can format for the Mac just fine. Installing Win7 just to get to an SSD is totally unnecessary, unless you just want to run Win7 for something else.

You use DaisyDisk to drill down to find the files. So continually click on the large sections of the pie chart and you will see the files that make up that portion and you should also see their sizes. Once you've identified the larger files that you can remove or backup elsewhere, you can likely have DaisyDisk open a Finder window at that location for you to delete/move.

This advice proved very useful and the SSD is now ready for use as a fusion drive. Much obliged to you for this.

Thanks for this Ashwin. I've double checked on the excess files - the mobile documents 57GB according to daisy disk are on iCloud drive, but when I check iCloud drive - it is empty!!! I think this 57GB is made up of mp4s mainly, but have no way of seeing WHICH mp4s - they could already be stored on my two external HDs - 2 plus 1 TB, but I really want to free up this 52GB to have more room to start editing footage on the comp in iPhoto. My SSD is ready for use, just need to tidy up these mysterious " loose ends". Jack

Did you try Onyx?

I did - but managed to use daisy disk to identify footage - called mobile documents which was duplicated elsewhere and made space that way Thx a lot Jack