This is from the official documentation of veracrypt. I guess it also goes for older truecrypt. Not sure if your answer is here:
Note: For system encryption, there is no backup header at the end of the volume. For non-system volumes, a shrink operation is done first to ensure that all data are put at the beginning of the volume, leaving all free space at the end so that we have a place to put the backup header. For system partitions, we can't perform this needed shrink operation while Windows is running and so the backup header can't be created at the end of the partition. The alternative way in the case of system encryption is the use of the Rescue Disk.
Note: A backup header (embedded or external) is not a copy of the original volume header because it is encrypted with a different header key derived using a different salt (see the section Header Key Derivation, Salt, and Iteration Count). When the volume password and/or keyfiles are changed, or when the header is restored from the embedded (or an external) header backup, both the volume header and the backup header (embedded in the volume) are re-encrypted with header keys derived using newly generated salts (the salt for the volume header is different from the salt for the backup header). Each salt is generated by the VeraCrypt random number generator (see the section Random Number Generator).
Note: For system encryption, there is no backup header at the end of the volume. For non-system volumes, a shrink operation is done first to ensure that all data are put at the beginning of the volume, leaving all free space at the end so that we have a place to put the backup header. For system partitions, we can't perform this needed shrink operation while Windows is running and so the backup header can't be created at the end of the partition. The alternative way in the case of system encryption is the use of the Rescue Disk.
Note: A backup header (embedded or external) is not a copy of the original volume header because it is encrypted with a different header key derived using a different salt (see the section Header Key Derivation, Salt, and Iteration Count). When the volume password and/or keyfiles are changed, or when the header is restored from the embedded (or an external) header backup, both the volume header and the backup header (embedded in the volume) are re-encrypted with header keys derived using newly generated salts (the salt for the volume header is different from the salt for the backup header). Each salt is generated by the VeraCrypt random number generator (see the section Random Number Generator).