That's what I thought but seeing your answer I repeated my test and I really think that X is stored on C drive.
The X folder properties is 165GB with only 13GB free (which is exactly the same properties as my C drive).
I copy/paste a set of folders whose total size equals 21GB into the X:\Google Drive folder.
I then receive a message from Boxcryptor saying :
"Insufficient space on Boxcryptor, additional 8GB required to copy these files
Free space : 13 GB
Total space : 165 GB"
If there is no way to change this limitation, then I will need to use another software.
Did I miss something ?