I also use Calibre Portable - Calibre and the Calibre Library are stored at a shared Dropbox-Folder.
I wrote a tiny "Calibre Multiuser Starter" Tool.
I did not start the Calibre-Portabe.exe, i use my "CalibreMultiuserStarter.exe" wich is placed in the same folder.
At Startup the tools Create a little "Lock"-file.
The fle is small and quickly uploaded by Dropbox and other Cloud Services.
If the Tool start an find a Lock-File the use will get an Error Massage that "User xy on Computer zz" have already start Calibre an quit.
If the use close the Calibre exe my Tool will delete the Lockfile and quit also.
Here the Link and the Source-Code in AutoIt:
http://znil.net/index.php?title=Cali...ool_/_Funktion
I also wrote a Version for Terminal-Server (catch the the Computer Name of Client) if anyone want to have.
Bernhard.