Buying a new 8 TB drive. The intent is to make a backup of a subset of data stored on my DAS.
The question is, should I rotate the NEW drive into the DAS (RAID 5, if it matters) and pull one of the older drives for my “archive copy”? My understanding is that I could swap the drive out and the array would happily rebuild.
Or should I use the new drive as my “lives in a box except for a couple times a year when I spin it up” extra copy of data?
Thoughts?
I’d say never do that. You would not have a copy of the data if something happened to the RAID. You would just have garbage on the drive you pulled.
Not what RAID is intended for. You also put a lot of strain on the other drives during rebuild. Especially if this happens regularly.
Just plug the spare drive into the USB or have a HDD dock to connect to computer and copy the relevant data from DAS to drive. And do this on a regular basis or use a script/backup software to trigger the copy when the drive is connected
It’s all backed up to the cloud, but I see your point - removing the drive is a “drive failure” scenario rather than a “do it because you can” sort of thing. 
If I were you, I’d retain the new drive for the backup.
Only for the reason that if a drive dies and it’s in the RAID (and there’s a notification or indicator), then you can recover by putting a new drive in. If your backup drive fails, you may not notice until you need it, at which point you’re back to your online backup.
Of course, per the Bathtub Curve Drives tend to die very quickly or after a long period of time. So either could be a greater risk (new drive or old drive)
So I’d set up the new drive for a backup and order a replacement for your DAS to put on a shelf, just in case.
I agree with @geoffaire. I had to deal with failed drives many times over the years and each time I sweated bullets until the rebuild was complete.
If you have a second drive failure when rebuilding a RAID 5 you lose everything. Just keep a spare on hand. 
If it’s mostly going to be sitting on a shelf and getting spun up maybe once a month, do you think the drive grade matters?
Looking at Seagate Exos vs. Ironwolf vs. Barracuda. Barracuda is much cheaper. Looks like IronWolf and Exos are rated for 2x and 4x the yearly TB, respectively.
Any thoughts?