Here is an actual e-mail from a Canon tech regarding this problem:
Thank you for your recent enquiry regarding your Canon product
Please accept my apologies for the delay in contacting you regarding the
problems you have been experiencing with your CanoScan D646U.
I shall firstly answer your query regarding the TWAIN drivers. There are four
main TWAIN files that are pre-installed with Windows, TWAIN.DLL, TWAIN_32.DLL,
TWUNK_16.EXE and TWUNK_32.EXE. These are included with the scanner drivers to
ensure that a compatible version of the files are installed, otherwise the
scanner will not work. During installation these files are checked for and, if
a later version is already installed, the versions from the disk will not be
installed. I can assure you that the scanner driver will work OK with version
1.7 of the TWAIN drivers.
For the scanner to communicate correctly the Still Image Services needs to be
running in the background. To check this please open the Control Panel and look
under Services in Admin Tools. Please check to ensure that the Still Image
Service is running correctly.
If all this is OK, I would recommend removing the driver in the following way.
With the scanner still connected go to Device Manager. Under Imaging Devices
right click on the CanoScan D660U and select uninstall. After this please
disconnect your scanners USB lead and remove the rest of the driver using the
DELDRV utility. Before reinstalling the driver you must restart your PC and
then reconnect the scanner.
If there are still problems please check that the driver has installed
correctly. The following files should be shown in the driver section when you
look at the scanner properties in Device Manager.
All of the files mentioned above that are listed as being in the TWAIN_32\D64U
folder are the exact files that should appear in that folder if you look in
A further cause of the TWAIN error occurring is if there is a conflict with
another TWAIN compliant device, such as a Digital Camera or scanning device
(including Multifunctional devices. If you have such a device installed try
removing its software before reinstalling the scanner driver.
We hope this information is of use to you. However, should you require any
further technical assistance, please do not hesitate to contact us quoting your
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Product Support Specialist