TrueCrypt was an on the fly encryption solution abandoned by it's former developers under the false presumption that it was no longer needed with the end of life of Windows XP. They claim it is replaced by BitLocker although that is only enabled on Enterprise versions of Windows. Professional doesn't have it although it has EFS. However programs like TrueCrypt are required for Home "Premium" and lower versions of Windows which lack EFS. These solutions are not practical for flash drives and memory cards which are not usually NTFS formatted or partitioned.
Also a cross-platform encryption format is required.