Hello, my friend, based on your description, I guess the problem may come from the SD card file system failure. You're suggested to repair the SD card via Disk Utility or format the SD card to give it a new file system.
Fix 1: Repair the SD card with Disk Utility
If there are some tiny errors with your SD card, you can run Disk Utility First Aid to repair the disk on Mac. The steps are just a piece of cake and you can do it without a hitch.
Step 1: Open Finder > Applications > the Utilities folder. Then, open Disk Utility.
Step 2: Click View at the upper-left corner and select Show All Devices to list all detected disks and volumes.
Step 3: Select the target disk or volume at the left sidebar.
Step 4: Click First Aid and then select Run on the popup window.
Step 5: Wait until the First Aid finishes running. (Don't do other operations on Mac when First Aid running).
If this fix fails to make your SD card readable, then I guess you need to format it on your Mac. Here are how to format an external hard drive on a computer:
Step 1: Connect the external hard drive with your Mac, and the computer will mount your drive.
Step 2: Launchpad → Other → Disk Utility, and click the Disk Utility.
Step 3: Choose your target external hard drive or volume, and click the Erase button on the top of Disk Utility.
Step 4: Edit the disk name, and select a file format and a scheme.
Step 5: After choosing the partition parameters click Erase at the bottom of the window to confirm the formatting of the drive.
Step 6: Click Done, and you have finished all the steps. Then you can quit the Disk Utility.
After the formatting work, you can head to check if your SD card can be read by your Mac and the external portable DAC.