I’ve been talking a lot about my backup strategy recently and how I have both local and cloud components in it. While we use Time Machine for regular backups of our Macs, I’ve been using external drives to store our photos for many years. I’ve got two RAIDs that I use for that purpose. As our storage needs continue to increase, I regularly evaluate the size of those RAIDs to ensure we have enough space. We’re not quite there yet, but the time is coming, so I’ve started doing some research.

A Tale of Two RAIDs

First off, what is a RAID? RAID stands for Redundant Array of Independent (or Inexpensive) Disks. RAIDs have existed since about 1988 and provide a way to combine multiple storage devices together to either act as one. There are two general configurations – striping and mirroring.

Striping is about performance: it treats two equally sized drives as a single drive, writing to and reading from each concurrently to improve throughput. Two striped 2 TB drives look like a single 4 TB drive, but perform faster than a single 4 TB drive. Mirroring is about redundancy: it duplicates data between two drives so that if one drive fails, the other takes over. Every single time a file is written, it’s written to both drives. Two mirrored 2 TB drives look like a single 2 TB drive. If one drive fails, it can be replaced with another drive and the contents of the first are automatically copied to it.

I bought my first RAID in 2010 – a Newer Technology Guardian MAXimus. It’s a dual-drive enclosure, encased in black anodized aluminum with small perforations for airflow, similar to the design of the Mac Pro from that era. The drives connect internally through eSATA and externally through FireWire 400/800, USB 2.0, and eSATA. I put two 2 TB drives in it and copied my photos over from their single external drive. During the life of enclosure, one of my drives failed and everything lived up to its promise. I replaced the failed drive with one of equal size, the enclosure duplicated the data, and everything was back to normal.

By 2018, I was starting to outgrow the 2 TB drives I had in my RAID. I started shopping for larger drives but considered if there were additional opportunities. I was starting to need external storage space for more than just photos and I decided to invest in an additional RAID instead of upgrading the existing one. I bought an OWC Mercury Elite Pro, another two-drive aluminum SATA enclosure (this time in silver), but with Firewire 800 and the new-fangled USB 3.0 for connectivity. I put two 4 TB drives into it and made it into my dedicated photo RAID. I turned my 2 TB RAID into storage for Mac software and other randomness.

Fast Forward to Today

I’ve been using my two RAIDs for the last five years in that configuration and it’s starting to become time to plan my next upgrade. I currently have about 1.5 TB left on my photo RAID, which will last at least a year, but I did a quick exercise recently to see what my options are.

Option 1 – Upgrade to 8 GB Hard Drives

This is the cheapest and most straightforward option. I’d replace my 4 TB spinning hard drives with 8 TB spinning hard drives, move the 4 TB drives to my Mac Museum RAID, and copy all the files over. I’d use the same enclosures I have today and double my storage capacity. For general use and backup purposes that would be fine, but it wouldn’t speed up file transfers at all.

Total cost: about $250

Option 2 – Upgrade to SATA SSDs

At max, my spinning hard drives can transfer 60 – 70 MB a second. Compared to the 5,000+ MB per second that my internal SSD can deliver, it’s pretty slow. A USB 3 connection can support around 600 MB/s, which many SATA SSDs can deliver. Upgrading to 8 GB SATA SSDs would provide the space as well as a performance boost. The problem? SATA SSDs at that size are about $700 a piece.

Total cost: about $1,400

Option 3 – Buy a Quad SSD RAID Enclosure

If two 8 TB SSDs are too expensive, what about four 4 TB SSDs? There is a RAID configuration that combines mirroring and striping – the drives are paired so that two are considered one, then they are duplicated across the pairs. I could buy four 4 TB NVMe SSDs and configure them to act like two 8 TB mirrored drives. The striping would provide up to 2,800 MB/s throughput over Thunderbolt 3, a little over 4x the performance of Option 2.

While faster, this isn’t much cheaper than Option 2. A single 4 TB NVMe SSD costs about $250, bringing the total for four up to around $1,000. A quad SSD RAID enclosure adds another $350. Trying the same configuration with quad 4 TB SATA SSDs only reduces the cost by about $20 per drive.

Total cost: about $1,400

Today’s Choice

It’s pretty obvious from this fancy graph that Option 1 is the cheapest choice, especially right now. I don’t think Option 2 will ever pan out, but Option 3 might in the future. Option 1 is also the least performant, but performance is a convenience, not a requirement. With the industry transitioning from SATA SSDs to next-generation NVMe SSDs, prices for 4 TB modules will continue to decline. I’ll look out for deals over the next few years and pull the trigger if something looks good. When I need the space next year, 8 TB spinning hard drives will fit the bill even if they don’t provide any performance boost over what I have today.