Mixed batches at least.
And, of course, backups. A RAID by itself is no surefire guarantee.
Luckily for us, a single 8Tb can hold all of our data. We are in the process of migrating over to a new server (new domain and server running server2k16), and our decision is to move only current data over to the new server, and leave the old data on the old server (which is, frankly, a nightmare to sort out).
Then the old server will be switched off for 3 months. After that, a new 8Tb will be procured, all data will be copied over and the old server be decommissioned (it’s running server2k3).
We most certainly don’t want old baggage data (domain controller included) to be copied over in the migration, plus it is to a new domain anyway.
Then the idea is to inspect the old server’s hardware, and if it is still good, repurpose it as a NAS running FreeNAS and use it as a fallback should our main NAS fail. This part will be interesting.