How to get my user data back? A few months ago, I partitioned my hard drive to install OS X Yosemite Beta, everything was fine but now that I'm trying to access to some data on my old partition user account (running on Mavericks), it looks like there is nearly nothing that remains, just some data in my documents and download folder.

I can't remember if the data was meant to be on this account, or if it was on another one that would have been accidentally deleted/hidden.
The only thing that makes me think that I could get quite easily this data back is that this partition has got lot of data, something like 80 GB, compared to the extremely small size of my user folder, which is only about 4 GB (and I don't think that the apps would take as much space as 80 GB)

So, have you got any idea to help me solve this mystery? :)
The easiest way to find out where all this data is, would be to go to the root of the partition in Finder, then hit Cmd+Alt+i. That will bring up the Inspector that should tell you that, yes, your partition has 80GB of data on it.

That's not particularly helpful in itself, but if you then select the folders contained within "/" that are visible ("Applications", "Library", "System" and "Users") you can see the corresponding sizes of those folders.

If "/Users" is huge, then it's possible that your data is still there but hidden for some reason. If it is, I would suggest rebooting to your Recovery Partition and resetting your User folder permissions and ACLs.

As a good measure, I would also repair your Disk Permissions from Disk Utility in the Recovery Partition too, just in case there's extra fluff getting in the way.