I take it your computer does not have an SD slot reader? Sorry, the answer seems obvious since you’ve tried an external card reader . . . .
Make sure your SD card is not locked (small slide shutter on the side of the card).
If you insert a flash drive in your USB port, do you see it on your desktop or in a Finder Sidebar window under Devices? This is where you should typically see the card or a flash drive. For example, my card is named UPSTAIRS in the screen shot:
With your SD card in the pedal and with the pedal power off, connect the USB cable to your Mac port, power the pedal back up. Does the pedal appear as a device? If still not, open Disk Utility and if the device appears there, take a screen shot and post it to this thread, please. Should look somewhat like this screen shot
If still no joy, when you disconnect the USB cable from your Mac, do you get a “Device not ejected properly” alert?
With the USB cable disconnected, power your Mac and your pedal down. Leave the USB cable (or card reader) disconnected. Power the Mac back up. Connect either the pedal USB cable or the the card reader and power the cable back up. Anything?
If none of these works, can you try the SD card in another computer? Either Mac or PC, doesn’t matter.