MacOS External Storage Setup

A combination of a growing volume of personal media from 24+ megapixel cameras/4k HD video-recording phones, the switch to smaller solid-state internal drives, and the increasing life-span of our desktop computers, means that at some point you are likely to find yourself needing to offload data to an external storage device. Having gone down this path earlier in the year I thought I would share my experiences with the WD My Book Duo.

I chose to go with direct attached storage instead of network-attached storage (NAS) for a number of reasons but chiefly performance and simplicity. In order to be available on-demand a NAS devices has a constant power draw, and unpredictable fan-noise making it a potential irritant for any other occupant of the room. Device transfer speeds are always quoted when using high speed wired networking too, and if the intention is to keep it in the same room and connected to the primary computer via a cable, the network-aspect is just adding management complexity and security risk.

The My Book Duo was easy to setup. I chose to set the pair of disks to mirror each other (RAID1) to ensure I could continue to work uninterrupted when a disk inevitably fails. Offsite backup—necessary in case both disks are lost, for example fire or theft—is to Backblaze. I also activated the device’s hardware encryption (called “password protection”) because I also planned to backup my internal SSD to this via Time Machine, and there is no point encrypting the computer only to leave off-computer backups open to all!

I formatted the drive using Apple’s modern filesystem, APFS. I thought this would be a forward-looking choice, and allow me to take advantage of its filesystem snapshots feature to make incremental backups of my files using a minimum of extra space. I quickly discovered that in Mojave (and Catalina) APFS cannot be used for a Time Machine backup so had to split my disk into two partitions1. Unfortunately it turns out that there is no built-in mechanism, not even a command-line, to create snapshots on an external drive. The only application that I can find capable of creating them is Carbon Copy Cloner, and even then it is a side-effect of taking a backup and not accessible as a standalone feature. The developer of that app has done a lot of interesting analysis of APFS which indicates that for certain usage patterns it is also noticeably slower than HFS+ on a rotating hard drive. That usage pattern includes browsing a lot of files, which is exactly what I am doing when I open up my huge folders of photos. 🙁 With this additional knowledge it seems I should have stuck with the venerable HFS+. Apple provides a tried-and-tested utility to convert from HFS+ to APFS, but not the other way, so starting with HFS+ would have allowed me to upgrade later once there were sufficient benefits to outweigh the hit to browsing speed.

However despite this suboptimal configuration choice, it seems the biggest error I made, was to install the WD agent software. This was actually necessary to configure the disk mirroring and enable the encryption, but once I was up and running I was constantly finding that the drive would unexpectedly disconnect while the Mac was sleeping. With password protection enabled this would manifest as the top level directories all appearing but then not showing any contents when double-clicked–a panic-inducing moment so I quickly disabled the hardware-level password protection2 in the hope it was just a bad interaction between MacOS and the WD software. That did stop the “missing files” but now MacOS would just complain at me that the disk had been disconnected without being unmounted. Worse, that unexpected disconnect would sometimes cause Finder to completely lock up and require a reboot–an incredible and scarcely believable failure mode in 2019.

Since this never happened while the OS was awake I guessed it was caused by the drive going to sleep and MacOS thinking it had disconnected. I tried disabling both the drive’s sleep timer, and also the one in MacOS to no avail. I even tried turning off the WD software agent, which resulted in a short trouble-free period but then just as I thought I had solved it, the problem would return.

Although I had disabled the WD agent from starting automatically, I did keep the management software installed in case I needed to tweak any settings. One day while inspecting running process in Activity Viewer I noticed that there was a running WD Agent process! I had allowed it to check and install software updates periodically, and I assume one of those updates must have reactivated the agent. That was unacceptable so I completely removed all trace of the software from the system, and not experience any unexpected disconnects since. Not having access to the WD management software does mean I cannot use the hardware encryption or easily monitor the hard disk health, but must instead rely on the LEDs on the front of the device to warn me when one disk has failed and I need to quickly purchase and install a replacement to restore my redundant mirror.

Now it is working correctly, the My Book Duo has been a good purchase and the setup works well for me. It came with both USB-A and C cables so I can also connect it to my USB-C only laptop which means copying files to it to work away from home is considerably quicker than transferring them over the wireless network. However it required a surprising amount of technical knowledge and experience to get right: the default configuration is RAID0 (meaning that all data is lost if either disk fails, doubling the risk) and the Agent software causing the drive to disconnect is disappointingly poor—does no developer at this company use their own product and find these bugs?

  1. I sized the Time Machine partition using the “2x the capacity of your source device” rule of thumb[]
  2. I switched my Time Machine partition to using MacOS’ built-in filesystem level encryption.[]

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