Backing up Bitlocker encrypted data

> My laptop data is encrypted by Bitlocker. As an FYI, the laptop does not
> have a TPM chip so I store the key on a flash drive.
>
> I wish to backup the data on a WD My Book. I understand that a standard
> back up will not be encrypted. So at the moment, the backup drives are the
> weak link in my data security plan.
>
> I think I prefer that my backup data also be encrypted by Bitlocker and
> ideally, with the same key as the original.
>
> I am hopeful the community will offer advice to improve my data security.
>
> Greg