Thank you Darac for your input. Here's an update on what I'm doing now.
I caused the RAID1 mirrors to complete the sync in a dropbear boot environment. Now I'm writing /dev/zero to the crypt mounted volumes. No changes yet to the form of the command, but if I see troubles, then I'll consider dropping block size and use cat as you suggest. There is a long way to go before the /dev/zero writing will be done, I'm going to get some sleep soon and check it later. Kind Regards AndrewM
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