Got a Mac with a SD Card slot? Don’t rely on it as a storage upgrade

Spinning disks are slow and solid-state drives (SSDs) used to cost a digital arm and a leg. That led many people to stick with hard drives or purchase Macs with low-capacity SSDs—like 250 GB or 500 GB—because the next increment up added many hundreds of dollars to the cost. (I’m sitting here with a 2017 iMac with a 1TB Fusion drive, so I am one of you.)

If you’ve got an hard drive in your Mac or a low-capacity SSD, you are surely tempted to update your system, adding speed or capacity. And some of you might be tempted to trim costs on that upgrade by using an SD Card (typically in the Micro SD format) inserted into the card slot present on generations of Macs preceding those that incorporated USB-C or Thunderbolt 3.

I recommend against the SD Card route, tempting as it may be, unless you’re using a card for largely static storage—like offloading files you want on the devices, but aren’t reading or writing—rather than as a boot drive or external active drive.

Gordon Mah Ung

SD Cards use the same flash memory chips as SSDs, but the way in which the memory is packaged and managed is quite different. An SSD has a more sophisticated controller system designed to work with the limitations of flash memory, which wears out after a significant number of write operations. An SSD “levels” this usage so that no single location is written consecutively or excessively. Leveling wear dramatically extends SSD lifetime. Many drives optionally offer “trim,” a feature in which the drive and and operating system pass information on file deletion that helps improve overall write speeds.

SSDs also have a distributed architecture for the flash memory chips that allows far faster speeds than SD Cards. A high-end 1TB Lexar HD Card that’s labeled 95 megabytes per second (MBps) for reading data and marked Class 10, U3, V30—three measures of performance—for about 30 MBps of writing data is just around $200 street price. An SSD from Other World Computing that can be installed in place of existing SSDs in the last generations of MacBook Pros with removable drives is $329 for 1TB—and has a rated 3,282 MBps read speed and 2,488 MBps write speed. Rather different.

Even if you can’t swap your internal drive, by the way, for a Mac mini or iMac, you could use an external SSD in a USB 3 or Thunderbolt 3-equipped enclosure. OWC offers a 1TB Thunderbolt 3 SSD for just under $300. You can clone your startup volume to the external drive, restart, and find your machine has a new lease on life. While this is possible with a Mac laptop, ensuring the drive remains plugged in wherever you’re using may be too stressful.

This Mac 911 article is in response to a question submitted by Macworld reader Marcella.

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