Upgrade - replace with larger HDD, in RAID6, two at a time?

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Hopchen wrote:

Even then, the NAS will still only sync one disk at a time as far as I am aware. 

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OK, thank you for attempting to answer one of the questions.

I should, perhaps, been more informative.

I have a Computer Science background & have decades of experience in the industry.

I HAVE BACKUPS. I have generally stopped updates (where I dont' they are minor & I backup again).

I am upgrading capacity.

I have done 2 of 6 drives.

Each drive, due to background priority, takes around 24 hours to resync.

I would prefer to get this over and done with.

ON the basis of MTBF, I suspect a reasonably low chance of a failure in the next few days. If not I can recover.

I had presumed that someone, particularly Netgear themselves, would have experience with a 2 drive RAID6 failure/resync scenario.

(where I would expect a resync - after shutdown/replace 2 drives/reboot - would actually resilver both in one go, that's how I would have designed it)

(I would expect the parallel IO to allow faster resync, than two sequential ones)

So I asked a couple of questions to confirm my thoughts.

So thank you for your concerns.

Should anyone have actual knowledge of the answers, I would be thankful for the input to my decision making process.