Between the two options, from my perspective, if the system is ONLY a backup of your main NAS, then the fewer drives, the better.
Let me justify it:
You run a greater risk of data loss using such huge drives during a RESILVER operation due to the days it takes to RESILVER a drive and the possibility to another drive failure, or worse, two drives. BUT, if this is only a copy of your main NAS, you could recover from it.
If you were planning to store other data that was not backed up elsewhere then I’d choose the smaller drives, for the same RESILVERing reason. But for the question you asked, 5 of the 28TB drives will consume less power, might be less expensive as well overall.
If your system allows for the physical addition of one more drive, you could keep a cold spare handy and when you do have a drive failure, add that drive in and and let it RESILVER and replace the failing drive. It is a little safer this way.
Don’t worry about how long a SCRUB takes, the system will run in the background and it’s a backup, who cares how long it takes. It only becomes a pain when you are troubleshooting a problem.
And lets not forget performing routine SMART Short and Long tests. For the 5 drive setup I’d run a Daily Short test (I always recommend this since it takes about 2 minutes) and a depending on the Long test duration, either once or twice every month. I’m leaning towards once a month as I suspect the duration of the Long test could be over 24 hours, but if the drives are spinning all the time, twice a month sounds better. Catch any obvious problems sooner than later.
What are the drive models you were thinking about?
Again, just my opinion. We all have them. You will need to decide what is best for yourself.
