BitLocker to Go volumes are fully supported on Windows 7 and later. For compatibility with systems running Windows XP or Windows Vista, a utility called BitLocker to Go Reader (archived) is provided to enable read-only access to the volume on these older systems.
The BitLocker to Go Reader,
bitlockertogo.exe, is included on encrypted volumes as an unencrypted file that is readable by older versions of Windows:
By default, BitLocker To Go Reader is stored on an un-encrypted part of removable drives that are protected with BitLocker. (Source)
However, if BitLocker is supposed to encrypt the entire volume, including filesystem metadata, how does it leave a file unencrypted and readable in this manner? Does this mean that it leaves a portion of the volume's filesystem metadata unencrypted for this purpose? (If so, how does it do this without interfering with the rest of the filesystem, and how does it ensure that the encrypted portions are not accidentally overwritten by the legacy OS?) Note that this does not work on NTFS volumes:
To use BitLocker To Go Reader on a computer that is running Windows XP or Windows Vista, your BitLocker-protected removable drive must be formatted by using the exFAT, FAT16, or FAT32 file system. If the drive is NTFS formatted, it can only be unlocked on a computer that is running Windows 7.