[KevinH]

All epub paths are bookpaths from the epub root so these should only use "/" as all hrefs do on all platforms.

Checked the actual code and I am happy that it handles things and only lets valid bookpaths through and \ is not a valid url or href character so I think the code does what it needs to here:

SelectFolder select_folder(group, m_Book, this);

    // Get the destination folder from the user
    if (select_folder.exec() != QDialog::Accepted) {
        return;
    }

    QString folder_path = select_folder.GetFolder();
    // Make sure folder_path is valid if abort
    bool valid_path = folder_path.indexOf("..") == -1;
    valid_path = valid_path && folder_path.indexOf("\\") == -1;
    valid_path = valid_path && !folder_path.startsWith("/");
    valid_path = valid_path && !folder_path.startsWith("./");
    valid_path = valid_path && !folder_path.startsWith(".");
    valid_path = valid_path && folder_path.indexOf("/.") == -1;
    valid_path = valid_path && !folder_path.startsWith("META-INF");
    valid_path = valid_path && folder_path.indexOf("META-INF") == -1;
    
    if (!valid_path) {
        Utility::DisplayStdErrorDialog(
	    tr("Destination Folder has invalid path \"%1\"").arg(folder_path));
	return;  
    }

If your destination already existed you would see its bookpath in the SelectFolder dialog and can see what types of paths it wants. The BookBrowser is not a generic File browser and as such uses / as its folder separator.