The dedicated iFly hardware devices are Windows CE-based. There are various limitations baked into that older OS that you don't run into with more modern setups. I'm having trouble finding definitive information on the file system limitations with WinCE, but it appears that that are at least some configurations that were limited to storage volumes no larger than 32GB. WinCE also only supports the FAT32 filesystem, not NTFS, so you need to make sure your card is formatted appropriately.
Since other users have had success using 32GB cards, perhaps you could try formatting your 64GB SD card as a 32GB FAT32 device, and that would probably work with the 740b. You just wouldn't be able to take advantage of all the storage space on the card.