I failed to set up a Truecrypt volume to use it as Google Drive folder.
How can I pass over it?
I have succeeded applying Truecrypt for Dropbox folder.
Its actually pretty simple, and common to most file syncronisation software - they can't or won't synchronize files that are being written to, or have open handles. Its a 'lazy' way to ensure file integrity, since they can be sure the contents of the file arn't being altered while its being written to. Since google drive is trying to synchronize a single file (your truecrypt volume) while its in use, its waiting for you to be done with it.
Between the nature of encrypted files and syncronisation services, periodically unmounting (and maybe creating a copy) is the only way to get a consistant, working backup off an encrypted file.