When attempting to use my CanoScan D646U scanner a message appeared saying “Cannot communicate with scanner. Cable may be disconnected. Check status. Scanner driver maybe closed” which is followed by the message “Unable to open TWAIN source. Please check connection then restart Toolbox”. I have confirmed that the cables are connected to the scanner and all appears to be in working order. I have tried to reinstall the scanner software, but when attempting to uninstall the scanner a message appears saying “C:\Program Files\Canon\ScanGear Toolbox CS\SGTBox.exe is in use. Please close the application using this file and launch UninstallShield again”. How can I regain use of the scanner? The computer is running Windows XP Professional.

Researching your question has revealed that quite a few other users are experiencing the same problem, and the resolution to the problem is unfortunately not straight-forward in most cases. You have attempted to take the correct first-step towards resolving this problem which is reinstalling the scanner software, so we will continue along this avenue of investigation. The error you are receiving indicates that the SGTBox.exe process is still running on the computer. The uninstaller cannot remove that file while the process is running, so it has been halting the uninstallation process. The SGTBox.exe process is related to the Canon Toolbox and presumably remains running in the Windows background so it can open promptly when the scanner needs to be used. To close the SGTBox.exe process so you can uninstall the scanner software, press CTRL-ALT-DEL to open the Windows Task Manager. In the window that appears click the “Processes” tab. Sort the processes by name and locate the “SGTBox.exe” process. Once found, select the process and click “End Task”. You will be asked to confirm the termination of the process, to which you should answer “Yes”. Once the process has been closed, go to the “Add or Remove Programs” control panel and uninstall the Canon software (particularly ScanGear Toolbox). Alternatively, you can also go to the “ScanGear Toolbox” program group on the Start menu and select “Uninstall” – both methods perform the same function.

Now you also need to uninstall the TWAIN driver so that this component can be reinstalled. It is very important that you carefully follow this procedure to uninstall the TWAIN driver. Go to the “Start” menu > “Control Panel” and open the “System” control panel (you may need to first click the “Performance and Maintenance” category). In the System Properties window that appears, click the “Hardware” tab and the “Device Manager” button. In the Device Manager window you should see an entry for “Other Devices” and/or “Imaging Devices”. Your scanner will be located within one of these items. Expand both items (by clicking the plus + sign next to the item name) and locate the scanner within. Then, right-click on the scanner entry and select “Uninstall”. Confirm that you would like to remove the device. Once the scanner has been removed close the Device Manager. Disconnect your scanner from the computer – this is extremely important. Finally, restart the computer.

Once the computer has restarted insert the original Canon install CD into your CD drive and follow the prompts to commence the installation of the scanner. Do not connect your scanner to the computer until prompted. It is vital that the drivers and software are installed before the scanner is connected, as not following this process could result in the same problem occurring again whereby the TWAIN drivers cannot be loaded. Once the scanner has been completely installed restart the computer and then check whether the problem has been resolved.

Should the problem continue, it is possible that the Canon utility is unable to find the TWAIN source. TWAIN is not an acronym for anything in particular, but is widely referred to as “Technology Without an Interesting Name” and provides an interface between computer software and imaging devices, such as scanners. If the Canon utility cannot find your imaging device you may be presented with this error message. The reinstallation of the Canon and TWAIN software should have resolved any misconfigurations, but it is possible there is still some missing information on the computer. In particular, where the TWAIN information (source) for your scanner is located. We need to manually verify the location of this information and then check it has been configured correctly within Windows. To check the location, open Windows Explorer or My Computer. We then need to enable the display of hidden files and folders (as the TWAIN information is contained in a system directory). Go to the “Tools” menu > “Folder Options” and click the “View” tab. Enable “Show hidden files and folders” then click OK. Navigate to the following folder: C:\WINDOWS\twain_32. Within this folder you should see a sub-folder, which (hopefully) is somehow identifiable for your scanner. For example, it may have the model number of the scanner (D646U) in the folder name. Don’t worry if it does not – in any case, you should only have one sub-folder within the twain folder (unless you have multiple TWAIN-enabled devices connected to the computer, in which case there should be something in the folder name which identifies which is the scanner). Write down the entire path of the scanner’s TWAIN folder. For example, if the folder name was CNQL20 the full path would be C:\WINDOWS\twain_32\CNQL20.

Once you have the path for the scanner’s TWAIN folder, we need to add this to the Windows system path. Go to the “Start” menu > “Control Panel” and open the “System” control panel (you may need to first click the “Performance and Maintenance” category). In the System Properties window that appears, click the “Advanced” tab and the “Environment Variables” button (near the bottom of the window). Under the “System variables” heading you will see a scroll-box. Find and select the “Path” variable and click the “Edit” button. A window will appear allowing you to edit the value of the Path variable. Place the cursor into the “Variable value” field and make sure you are at the end of the text already populated in the text box (you can check by pressing the right arrow key on the keyboard, to see if there is any more text to the right of your cursor). Once you are certain you are at the end of the variable value enter a semicolon (if there is not one there already). The semicolon character is used to separate the individual values within the variable value field. Then after the semicolon, type the path to the scanner’s TWAIN folder. Make sure there is no trailing slash on the end of the path.

Using our above example, if the original variable value text was:

%SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem

The new variable value text would be:

%SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem;C:\WINDOWS\twain_32\CNQL20

Once you have modified the system path variable, click OK to save and close the system variable value then click OK to close the main Environment Variables window. Finally, click OK to close the System Properties window. Restart your computer so the changes take effect and then see whether the problem continues.