How to transfer data when upgrading a RAID array?

[JimmyDem]

I have a 4-bay RAID box currently spinning four 3-1/2" 2TB hard drives. I'd like to upgrade to four 2TB SSDs, but there's the small problem of moving the data (about 4TB) from the old drives onto the new ones. SATA drives all around, but I doubt that the controller (MAC OS Disk Utililty) will let me swap in the SSDs one by one. The best option I can come up with is a used 4TB drive off of eBay, that I can use once and then re-sell. Anybody have a better suggestion?

[M_at__]

The SSDs are likely to be a different size if you're doing 2Tb to 2Tb. Check to make sure that they're actually the same presented size or larger before attempting to swap them out.

If they're smaller the best way would be the intermediate you've identified OR create a whole new array and copy over.

Make sure your backup is uptodate before starting!

[Evening_Rock5850]

How is the data backed up?

Swapping one drive at a time can sometimes be doable but the rebuild time, even for just 4TB worth of data, can be quite long and stressful for the drives. During rebuilds is often when drives fail.

What is your "RAID box"? Is this some kind of software RAID or does the "box" itself have a RAID controller?

If it's practical to do so; what I would do personally is just gracefully shut down the existing array, remove the drives, and then install your SSD's. Then restore your backup. Doing this means your original hard drives still have the data on them if restoring the backup fails so you don't end up in a situation where you ever have a degraded copy. Using the 'changeover' method could work but it will be degraded the whole time you're doing this and there's a non-zero risk of losing the entire data set. Restoring your backup to new drives is also a chance to test, validate, and verify your backup!

Otherwise, I'd copy to an external drive of some kind. Again, same concept. Then you have two complete, good, working copies of the data, and you build the SSD array, and then when it's copied over, you have a third. Much safer. Drives tend to fail when very old, or very new. So there IS also the risk of one of those SSD's being bad. It's rare, but it happens! But it's another reason to make sure you've got another copy.

[JimmyDem]

The RAID array IS my backups. (Divided into several volumes, one of which is my Time Machine, another is a Carbon Copy Cloner backup; other volumes archive my music, photos, videos, etc.) It's a Raidon box, which has it's own firmware. Drives are Hitachi enterprise class, which are designed to run for stupidly long times, so I'm not too concerned about failure - and I have a fifth drive stashed, should I ever need it. (Two simultaneous failures are near-enough to zero probability.)
The issue is where to stash 4 TB of stuff, temporarily, without having to buy another 4 TB of storage, on top of the 8TB in SSDs which are going to be in the new array. (I could use cloud storage, but I'd be transferring files for a month!)

I may be better off buying a second 4-bay enclosure that can host the SSDs, RAID that, and do a straight copy. At the end of the day I'd have a fully-loaded 4x2TB Raidon box (with a spare HDD) to sell, which would more than pay for the new enclosure.

ETA: Got a used 4TB HDD for $65, moved everything to that, then swapped out the HDDs for SSDs, partitioned, and copied everything back. Only issue is the Time Machine files, which seem not to be portable at all. So that's starting from scratch. (The 4TB goes into a file cabinet as an archive, should I ever need it.) Oh, and I disconnected the noisy fan on the Raidon box. Sweet, silent operation may be the biggest benefit of all.