Two hard drives show wrong free space in ML, SL and Daisy Disk

Hi

I have a Mac Pro with four drives - two SSD (Boot and Video) and two WD Cavier HD.

Firstly, the amount of free space left on the drives is completely different between Mountain Lion and Snow Lion (need to use some of my scannng hardware) on one fo the HD is 200GB difference between the OS's.

Macintosh HD (SSD) drive with ML on it shows 127.18GB with 42.24GB free however this is only 15.9GB free in Daisy Disk. One of the WD HD is 1.5TB with 611.47GB free but in Daisy Disk is 262.6GB free. This was the drive with a 200GB difference when used with Snow Leopard yesterday.

Any ideas why the difference in free space and how I can correct this. I am trying to go through my HDs and get rid of unwanted / non longer needed files. But the difference in sizing makes it difficult to see how much work I have to go through.

Thanks in advance.

Matt