Paul wrote:

how much benefit does RAID provide, if you still
have to make backups ? Some people have the funny
idea that a RAID1 mirror is a backup. When it isn't.

Oh... sheepish look on my face ... yeah ... who would have a funny idea
that RAID is for backups ... not me (anymore anyway).

OK. So I won't bother with RAID. I will just make the new disk a Linux
disk, and dispense with grub to use this plan of action (after I back up
the data on the old disk).

1. I'll keep Windows on one HDD and put Linux on the other HDD.
2. I'll connect one of those HDDs to SATA port #1 on the motherboard.
3. Whatever HDD is connected to that port will always be the boot drive.
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A. If I press escape during startup, I can select the other boot drive.
B. If I find myself doing that a lot, I'll swap SATA cables on the MB.

That's the simplest I think, and it doesn't require grub.