Since you've transferred all photos to another external hard drive, you can try formatting the SD card on your camera instead of on the computer. However, if you want to solve the SD card not mounting on Mac issue, read the following guidelines. After mounting the SD card normally, you should be able to format it on your Mac.
How to troubleshoot SD card not mounting on Mac
Option 1. Connect the memory card to Mac via an SD card reader.
Did you insert the SD card directly into the built-in SD card slot? If so, you may try connecting the memory card with an SD card reader. Sometimes it's the improper connection that stops you from mounting the device.
Option 2. Try mounting and formatting the SD card on another device.
Possibly, it's not the SD card's fault but your device's. Try switching to another computer and see if it works.
Option 3. Manually mount the SD card.
As what's mentioned in your description, there is a Mount option when you can't get over unmounting. Try manually mount the memory card, is it working successfully?
Option 4. The file system of the SD card is not supported by macOS.
macOS only supports limited file systems, say, APFS, HFS+, exFAT, and so on. If you didn't initialize the external hard drive on Mac with a proper format, your Mac may fail to recognize your SD card. In this case, you can either format the SD card on a Windows PC or on your camera with a more versatile format such as exFAT and FAT32.
Now you should be able to mount it and format the SD card on Mac.