Thanks AdilsonColombo for your reply,
as your correctly point out, the message AWSHTC071E Host message text: Requested device number is offline says the volume (device number to be precise) is offline. However, hckd2ckd utility exits after displaying this message.
Contrary to your suggestion, I assume that "Migrating offline z/OS volumes from a z/OS system" is not possible.
Giovanni Creato