The danger with RAID with larger drive sizes is that rebuild times take longer. The pace of capacity growth has far exceeded the pace of drive speed increases.

Whenever a RAID5 array experiences a failure, the entire rest of the drives must read every single sector correctly to reconstruct the failed drive. And since RAID is a block level operation with no concept of the filesystem, no shortcuts can be taken in unused space. If hard drives had TRIM support, this could help reduce RAID rebuild times.

The larger the array, the higher possibility of a second drive failure during a rebuild. Especially if the drives are from the same manufacturing batch, and suffer a similar fault due to a worn out component.

The really bad situation is when the RAID software or hardware never does a health check of the array. It's possible a bad block has been sitting on some drive, undetected because the file hasn't been read in months. Another bad block on a different drive comes up and is seen due to it being active, the RAID fails the drive, then explodes when it hits the earlier undetected bad block. For ReadyNAS users, make sure "Disk Scrubbing" under Volume options is on to avoid this. SmartArray cards from HP default to doing passive checks when active array use dies down.

Some of this risk can be mitigated. RAID 1+0 results in far less drive activity across the entire array to rebuild a failed drive. RAID 6 offers the ability to recover even if 2 drives tank before a complete rebuild. But it can still be a gamble with larger SAN sized installs.

These alternative redundancy models sometime offer benefits, such as being file based instead of block based. This will ensure repair of a bad disk runs much quicker.