It’s tempting because of the price, but you’ll regret the performance.
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.
Read more at MacWorld.com