Follow the steps below to scan an SD card, USB flash drive, or another type of removable media on a Mac:

1. Install free Avast Security by following the instructions here: 
2. Once Avast is successfully installed, Go here and follow the instructions under "Run a scan". When you get to Step 2, choose the option "D. USB/DVD Scan" and you should be able to select your SD card or other removable media. If you can't, go back and select option "C. Targeted Scan" and select your SD card or other removable media from there. Then proceed to the next steps on that page.
   1. NOTE: do this for each SD card, USB flash drive, or other removable media because a later step involves you uninstalling Avast Security at which point you will no longer have the option of scanning removable media.
3. If you discover one or more infections, then follow the steps in the article I linked to in Step 2 above to remove those infections. If all of your drives are clean, then proceed to step 4 below.
4. Once you've scanned and cleaned your removable media, remove Avast Security by following the instructions on this page.
5. Reboot your Mac.
6. You're done!