While you may use FTPSync on any drive and directory on your system, FTPSync uses system's temporary directory (usually C:\Windows\Temp) for temporary storage during transfers. This sometimes becomes a problem, if you transfer large files and your system partition does not have enough free space. In this case transfers may fail, even if you have more than enough free space on your destination drive.
This issue has been resolved in FTPSync 2.8a.
After upgrading to this or later version, you will need to add a new parameter TempDir to Common section of your INI file. This parameter specifies which directory should be used by FTPSync for temporary files.
Example:
[Common]
TempDir=E:\Temp\