PS is quite selective in the format of an image. If all else fails open in another program, like Infraview, then save as TIFF. That ususally works.
But agree you need to fix the scanner. You may have to contact Cannon for them to rewrite the code. If it is indeed a bug they would surely want a fix as a lot of users have PS on their computer.
TIFF has many possibilities for bit depth, encodings, etc. plus the possibility of custom encodings and tags.
A few imaging vendors have abused this flexibility to include non-standard data in TIFF files while claiming that the files are interechangeable (I still swear they hire interns to write their file format support, and the interns never read documentation).
Anyway. Without a sample file to examine, I can't tell you exactly why Photoshop is rejecting it.
If you can provide a sample, then I could take a look and see what's going on.
Thank you all for your help.
The scanner options are to save as JPEG/Exif, TIFF or BMP (with standard, low compression or high compression).
I've just scanned a colour photo and saved it in all three formats (standard compression). All the files now open in PS, with the Camera Raw preview screen (which I've never seen before!).
I've also scanned a black and white drawing, both as a B&W photo and as a B&W document, and saved these scans as TIFFs. Both these now open in PS. The older files that I have not been able to open in PS still do not open. (These files open in MSoft Picture Manager and Quicktime Picture Viewer - though the quality is awful in Picture Viewer.)
It seems to me that whatever was causing the problem might have been 'cured' by some recent software update.
Do you think this is the likely answer? If you'd still be interested to look at one of the rogue files then I could upload one.