Yes you are right the free space I see in finder is the one free on my system drive.
I have two physical hard disk, system and data, the finder reports the free space of the physical drive I am looking but when going to iCloud drive it reports the free space of system and not the free space of iCloud drive.
That is strange because when I copy files to the iCloud drive the free space on my local drive does not change and so reporting the free space of my local drive does not make sense since it does not matter.
The problem is not only the display of not pertinent data, all my trouble with the size started when I tried to copy to iCloud drive a directory present in my data drive with a size of about 50 Gb.
The finder mumbles a little, I am using Yosemite GM seed 2, then says it cannot copy the directory since I don't have enough free space on iCloud drive.
I guess the solution could be to have 50 Gb of space free on my system drive or to copy 30 Gb at time (not exceeding my current free space).
Perhaps that could work but I find it a bug of the OS.
I have 176 Gb free of iCloud storage so I expect to be able to copy files to the storage without having to manage local free space.
My iMac has an SSD of 256 Gb and a data drive of 1000 Gb without fusion, so I have a lot of free space on my data drive, why should I bother with the free space on system to be able to copy files to iCloud?
Do you know if iCloud drive can be moved from the system HD to the data HD so to have a lot of free space on it?
I don't think the iCloud drive is mirrored on my local HD, it should be cached. Apple sells mac with 256 Gb of storage, if you buy 200 Gb of cloud storage and fill it with data, the mirroring will eat all of your local space, how could the cloud storage be useful this way.
Also on iOS the iCloud drive is only cached, since there is no local space for mirroring.