Agreed. The raid shouldn't be your only copy anyway.
Yup... only reason I don't go raid 0 for the extra space is because downloading 40TB of data takes longer than replacing a hdd and letting it rebuild
RAID 5 is fine, as long as you're aware the parity is next to useless on drives that size and you're almost guaranteed a URE on rebuild of the array. RAID is not a substitute for backup anyway.
Codswallop... it's quite rare for a rebuild to fail - even on disks of this size and even larger arrays.
I've only had it happen to me once in my life and that was because 2 of those nightmarish Seagate 3TB drives decided to fail within a couple of days of each other. Genuine hardware failure you're not going to get around.
I'm a Senior Network Engineer... so I've seen my fair share of arrays, array rebuilds and in-action upgrades/expansions which rebuild as you add the larger disks.
The only time you should worry is when a type of hard drive is nearing the end of its duty cycle.
I am just about to build a NAS with 4 x 4TB Ent drives. Should I go for raid 5, 6 or 10???
What's your usage scenario?
For home storage... raid 5... just keep any really precious data backed up to google drive / dropbox just in case.
The actual chance of a second drive failing before you've replaced and rebuilt a faulty drive is very low.
If it's professional storage... say for example you're a professional photographer and it's going to be where you store you main image backup - then go 10.