size of OSX Yosemite, don't understand who only 40 GB let on HD I installed Yosemite 10.4 on my 500 GB MBP hard drive.
I only have 40 Gb left.
Though i have :
Pictures 88 GB
Music 60 GB
Movies 5 GB
Documents 4 GB
Dropbox 6 GB
Desktop 0 GB
Downloads 1 GB
for a total of 164 GB out of 500 GB.
I have 40 GB left so this sum up 204 GB.
What is taking the rest of my space 500-204=296 GB?
Can't imagine that Yosemite+Recovery HD takes 296 GB inlcuding Library and System folders.

Please is there any explanation? 

Thanks!
DiskInventoryX will allow you to view a diagram of every file of your Mac and parts will be larger relative to file size. You can find it here: http://www.derlien.com/ This should allow you to find what's taking up so much space on your Mac.