I solved my problem by moving Calibre to D: and creating a symbolic link with junction.exe. (I looked into this method because I wanted to free up space on C: by moving chrome.exe and its 300 MO of files to D: !)
Here's the method (copy pasted from a site... whose address I have lost...
):
Go to the Microsoft website "SysInternals", and download the "Junction" file on this page:
http://technet.microsoft.com/en-us/s.../bb896768.aspx.
Extract "junction.exe" to C:\Windows\System32. Junction is a command-line program used to make "junctions" in Windows.
Close the program you want to move. Copy its folder from C:\Program Files to wherever you want it. Rename the folder in C:\Program Files to something else -- add "_old", for example.
Right-click the Desktop Windows button and select "Command prompt (Admin)".
At the command line, type:
junction.exe "C:\Program Files\Calibre2" "D:\Win_10_apps\Calibre2"
using whatever folders and paths are appropriate for you.
Launch the program (using the same shortcut icon that you were using before the move!) to test the validity of the junction. If the launch is successful, delete the "_old" folder.
Very useful!!
