That's not really HA then, it's just durability and replication, but at the storage layer. HA typically means high-availability during failure, so running multiple instances that are able to takeover with no (or very minimal) downtime.
You can certainly have very durable storage using your storage cluster with Redis just being an application that writes and reads from that storage, but there's still a risk of data not making it to disk before a failure of the Redis application. Also yes, you would have possible significant downtime since if the Redis server fails, you have to start up a new server, point it to the existing disk and then wait for it to load all the data into RAM again.
Replicating data with a secondary means you get read scalability since you can read from the replica, and it's probably safer since the network is usually faster than disk, meaning your data will be on the replica before it's written to disk on the master.