Retro Future Electrics

A page about retro electronics, from the introduction of electricity until 1970-something. We refit old gadgets with new tech, and wonder at the makers who came before us.

Fusion drive with internal & USB SSD on 2019 27″ iMac i9


UPDATE 1 July 2020: So after about 6 months with this setup I can’t recommend it. There were a lot of little strange errors. I ended up to buy the ifixit imac opening kit, put in a 4TB SSD internal and make that a fusion drive. After 3 weeks I’ve had no issues. What kind of issues did I have with the other? little things, it mostly worked, but sometimes files would be corrupt, and sometime the computer would report free space incorrectly (programs would crash with “no disk space” errors, but there were 500G free according to “df -h”)

Original Article:

When the 2019 iMacs were announced I quickly put in my order for a new machine. I anticipate using this iMac as my main computer for 5+ years so I put in some money into upgrades, including using only a SSD drive, in addition to upgrading to the best processor offered the 8-core Intel i9. As Apple wants a truly exorbitant amount to add solid state storage to their machines, I only ordered 512 GB. I thought this would be fine as I also ordered a 1 TB external SSD drive for much of my storage, and an 8 TB old-fashioned regular hard drive. This total amount of storage was fine, but I did not know that iCloud required it’s directories to be on the primary disk. This means that my computer would not store of the entire contents of my documents folder on the local computer, as my documents folder and my photos were larger than 1/2 TB.

At first I thought to order an internal SSD drive using the SATA standard to supplement. But I quickly realized that because I had ordered the machine with only a SSD drive, it did not have the required SATA cables. In addition opening up an iMac and replacing the hard drive is a bit of a chore requiring cutting tape and then resealing it with a kit that cost about $10.

The external SSD drive I had purchased runs on USB-C at 10 Gb/s, and in speed tests gives me about 900 MB/s of storage throughput. This isn’t his fast as the 2500 MB/s of storage throughput that you get from the built-in SSD, but it’s still faster than the 6 Gb/s SATA limit that I would have experienced with an internal SATA drive even if it was very fast SSD. I started to search the web to see if I could make a fusion drive with my internal and external SSD drives, initially I saw some negative results were people and had trouble with this configuration. However it soon turned out that this would work perfectly and I’m now very happily using this configuration with a 1 TB external SSD, and an internal 512 GB SSD. The fusion drive reports very first read and write speeds, in fact the rate speed is a little faster than the internal by itself as I believe it gets a little bit of RAID zero like striping bump.

Below are the step-by-step instructions that I followed to get this configuration to work, be sure to have a back up first as this process will 100% for sure delete all of your data on the drives that you’re using.


The following is a list of parts that I used for this, the links are to Amazon, and I get a small percentage of the sale if you purchase from them; however, I purchased all these items with my own money and the referral bonus does not influence this tutorial.

That’s it! All of the rest of the items are already built into the iMac. Be very careful that you but the right SSD and enclosure, as some are keyed differently due to different protocols.


The procedure is fairly straightforward, the basic idea is that you will wipe both of these hard drives and format with HFS, and then make a new fusion drive with a APFS. The only difficulty is to make sure that you know exactly which drive you’re working with, so that you don’t accidentally delete data on a drive you don’t wish. To be safe I would unplug all other external drives and then work only with your internal drive, and the single external drive that you’re using to make the fusion drive.

I followed the instructions exactly from this article and they were spot on. I can confirm this is working with macOS 10.15.1 Catalina. I wanted to write this article to confirm that it all works well as there were serval articles about previous versions that said it was unstable, but I’ve been up and running for three weeks with no problems.


On the whole the set up has worked perfectly for me for three weeks, however there is the problem that if you happen to bump the USB cable and disconnect the drive your machine will hang. I have only done this by doing it on purpose, as the USB-C cables are very secure. But just to be aware that you must be careful if your drive becomes disconnected easily due to knocking about then it will not work properly.

The other downside, which is true with any kind of fusion drive, is that if either drive fails the entire drive will fail. This means that you must have a good back up, because your chance of drive failure is now roughly doubled due to the double disks supporting your data. I’m super paranoid about backups after losing critical data in the late 90s. I now have 2 time machine backups and I use Backblaze for cloud based backup. I haven’t lost any data since around 2001. The cloud backup also provides a bit of protection from ransom ware, as it stores 30 days of changes.


I have been extremely happy with this set up. The iMac shows “Macintosh HD” as a 1.5TB fusion drive which is enough to hold my documents and photos locally and sync them to the iCloud. The computer seems a bit snappier as the fusion drive “magic” obviously includes some simultaneous writes and reads from both drives. I have seen zero instability, no grey screens, kernel panics, or even slight hiccups. In every way my iMac is running as before just a little happier.

Leave a Comment

Your email address will not be published. Required fields are marked *