You've certainly done the right things in narrowing the fault down so that's a great help.
Have a look down into the CF card slot, there should be two perfectly
even rows of contact pins (two rows of twenty-five each). If any are
bent (usually flat) or if there are gaps or obviously broken pins then
you have found your fault. If so, the camera needs to go into an
authorised Canon service centre to have the CF contact assembly
replaced and you need to check your CF card to ensure that there are no broken pins left inside it (probably not as you've already used it in other cameras, but double-check anyway). Unbending a flattened pin usually breaks it off or just leaves it weakened sufficiently to break off in the near future; the broken pin can end up stuck in the CF card, and when you insert the card into another device the blocked hole bends or breaks the pin on the next device.
If there's no obvious damage then to completely eliminate the possibility of the CF card being faulty it's worth giving it a full format in either another Canon EOS model or via an external card reader (remember to back up any stored images first). If that fails, try to borrow another CF card to check whether that works in your camera. If not then the fault definitely lies with the camera.
At this point the only remaining option is a camera reset but it's unlikely to work. Your camera must be in the Creative Zone mode (P, Av, Tv, M, or A-Dep) to do this as all menus are then available. Go to "Clear Settings" in the camera menu, you'll see how to do this in the owner's manual. A word of caution: it's been a long while since I did this myself, so I don't recall whether this resets the camera image file numbering back to zero. If it does then every time you try to upload photos from the CF to your host pc there will be usually be messages warning you that the file(s) already exist(s). This means that you'll have to rename everything. To overcome this, you need to find a jpeg (picture) file, preferably one taken with your camera, and rename it to the number before where you want to restart from. So if you want the new files to start at IMG_2000, rename the file to
IMG_1999). Then, using an external card reader drop the renamed jpeg file onto the CF
card in the 100CANON folder (the number actual number may be anywhere from 100CANON up to 999CANON). Any further photos will be taken from the correct number onwards.
If all of the above options fail then your camera must be sent to an authorised Canon service centre for fault diagnosis and repair.
If you're in the UK then click here for the UK service centres. Repairs will be free if your camera is still under the Canon warranty, but as it's only one year from the date of purchase it's likely to have expired on your 400D. The service centre will make a diagnosis and inform you of the likely repair costs; if you choose not to go ahead then the camera will be returned to you after you pay a small fee for postage and for the work already undertaken to investigate the fault.
Good luck with this and I hope that you soon have your camera working again. I hope that my reply has been of use and that you please spare a moment to rate my answer.
