1) The CompactFlash card may have ever been formatted with other DSC than DiMAGE 7.
Some DSCs format the card in their unique method that DiMAGE 7 cannot read. Reformat the card physically on a PC.
2) DiMAGE 7 is designed to check the capacity when a card is inserted. In case it finds any problem in the capacity data, the camera will not accept the card. Usually this occurs if the card has been used repeatedly and may have a part of information destroyed.
DiMAGE 7 cannot read the card if it has been initialized on some DCSs that executes a unique partition management. (The card is recognised the same as above.)
Performing physical formatting using a PC or some DSC capable of the function may solve the problem, but the optimized segmentation enabling quick access will be lost at the same time. DiMAGE 7 can format the card only logically to maintain the optimized segmentation originally provided.
http://www.minoltaeurope.com/pe/digital/compatibility/e/d7.htm