This is not a definitive answer, to be sure, but I would look at rsync to do the data-copying. If you're starting with a blank target disk, it will copy everything over. If, however, you're starting with a disk that has part of the data already in place, it will ignore files that are identical on the source and target, update those on the target that are older, and add any from the source that are not on the target at all.

I don't know of a down-side to rsync, but maybe more experienced users will offer more input.

~ William

On 09/03/2017 02:59 PM, Eben King wrote:
[log in to unmask]">I have a RAID-1 setup (2x1 TB) that I use for media storage.  I want to migrate to a new(er) set of disks, since the ones it's on are practacally antiques at this point.  Being smarter than the average bear I copied some of the most critical content (maybe 1/3 of the total) to an external HD, so if the entire thing goes tits-up I still have them.  I just got a 2-bay USB3 dock so I figured I'd use it.  One disk was copied successfully, at least about 2/3 of it.  Then something crapped out and I had to resume.  Assuming I got if= and of= correct in dd, the rest should be copied successfully too.  The other, I know I screwed up on.  I figured it would happen eventually.

So, what's the way that puts the least stress on the drives to expand the RAID from one disk to two?  Also, what's a more, er, "user-tolerant" way to duplicate disks that can be resumed?