I can not see the SD card sign in the 'finder' when inserted, but am getting the warning message when I take it out. I can't read the SD card now. I tried restart as well
Make sure that you have set finder to show external disks. Go to Finder preferences and then Sidebar. Then under Devices, check everything.
You've most likely corrupted your SD card, if it shows up when you insert it, try repairing it in Disk Utility.
Retrieving data ...