Opinions on Storage Upgrade options

[GeekStreetSolutions]

So I originally built my NAS with 6TB WD Red Drives using RAID6 (I believe these were the highest capacity then). I might have a drive that's failing, and I'm testing it now. My original plan was at one point upgrade all the drives at once to a higher capacity. However I have been thinking about it and I really have a two reasonable options:

1. Save up the roughly $4,000 and buy the drives all at once and upgrade when my storage is full.

2. Upgrade the drives as they fail with ones of higher capacity ones. Capacity will be based on the smaller 6TB drives though. Keep doing this over the years until all are replaced with higher capacity drives. This would also mean that all the drives would have different powered on hours compared to if they were installed at the same time.

Third option would be to buy both new drives and a expansion bay, but that would be costly, so I am avoiding that option.

What do you guys think would be the best scenario?

[dolbyman]

answer would be based on your budget and storage needs

have the cash now? need the space now?

don't forget to also grow your backups..raid is never a backup