The limit on FAT32 filesystems is generally 32GB. Anything larger uses something else and for cameras it is usually exFAT format. But the 50D pre-dates the existence of the exFAT filesystem so it doesn't know how to deal with those cards. Card sizes are typically powers of 2... e.g. 8GB, 16GB, 32GB, 64GB, etc. but I don't think anything bigger than 32GB will work on a 50D.