>system Bitlocker encrypted
>want auto unlock at boot but no TPM module
>use USB flash drive to auto unlock it instead

Is basically just a TPM substitute? As safe/'safe' as TPM? I.e. it will automatically unlock the system disk as long as it is still in that computer, but if you take the disk out, you won't be able to unlock it elsewhere, even with the USB?

Or is it super unsecure, is it basically like having the entire encryption key on the USB stick, and if a person takes out the drive and takes it with him together with the USB stick, he can then go to another computer and unlock the drive using the USB?