MBR shadowing
The shadowing feature
MBR shadowing is disabled by default, even if the device supports it. If shadowing is enabled, power cycling the device will make it show the shadow MBR instead of the actual disk.
Current shadowing state
You can manually switch between showing the shadow MBR or showing the disk contents. This setting is useful for testing, but, normally, you will only change the shadowing state from the PBA.
Uploading a PBA image
Uploading a PBA image onto the shadow MBR will make it possible to unlock the drive before booting the operating system. The PBA is a small bootable application that asks you for your password, authenticates to the drive, unshadows the MBR, and unlocks authorized locking ranges. In order to use the PBA, enter your BIOS or UEFI settings while the device is locked, and set the PBA as the first boot option.
You don't have to use SEDManager's PBA, SEDManager will happily upload any other PBA environment or even a cat picture. Make sure the PBA's password processing is compatible with SEDManager.
Linux has always been famous for how user-friendly it is. Unfortunately, when running as root, the file open dialog sometimes just doesn't work, and there is not much I can do about it. You can use the good old copy paste method to specify a path.