Due to the age of the camera, the Ultra 2 CF card may be using a different architecture than your other CF cards. In other words, the card is incompatible with the camera.
One other problem may be that the camera is not designed to recognize a card which is of a larger capacity.
As I cannot find the 300D on Canon's site it is difficult to find exact specifications as to the abilities of the camera, as to what type of CF card it will accept, what the largest capacity it will accept, etc.
A more fixable problem would be either that the card is not formatted properly - which may be possible through the camera itself. Please try to navigate the camera's menu to find a format option. If not, you can insert it into a CF card reader and attempt formatting it that way. Please see Here
for formatting through a camera.
Also, try Here
for yet another possible explanation of your problem, as #6.
If the camera is meant to take the capacity and type of CF card, and is not damaged, the only other explanation is that it is write-protected. As I do not know the exact features of your CF card, I cannot say whether or not it has a manual write-protection switch. If it does, it would be a small sliding piece of plastic or metal on the side of the card which toggles the write-protection feature. It may also be write-protected as a file system, in which case you will need to format it.
If the card refuses to format due to a write-protection, you will need to remove the write-protection from the card first and then format it.
The following picture shows a CF card with a write-protection switch on the left hand side.
If this proved helpful, please rate. If you need further assistance, please come back and I will attempt to provide further help.
Thank you and take care,