First, check your Finder Preferences. Open Mac Finder, and click on Finder from the top-left corner status menu bar. Select Preferences > Sidebar, under Locations, and tick the box next to External disks. Switch to the General tab, and choose External disks under Show these items on the desktop. Now, check if your USB storage device is showing up in Mac Finder or desktop.
If the USB flash drive is still not detected in Mac Finder, maybe the flash drive is a little damaged due to the unsafe ejection, try using Disk Utility First Aid to repair it and then check if the problem still exists. If First Aid failed to fix the USB flash drive not recognized on the Mac issue, the last resort you can try is to reformat it.
But reformatting will wipe all data off the flash drive! Therefore it's recommended to recover the important files from the USB drive with a data recovery tool first, here we recommend iBoysoft Data Recovery - a professional USB drive data recovery software.
Three simple steps to recover data from a USB flash drive that's not recognized on Mac:
Step 1: Download install and launch iBoysoft Data Recovery for Mac.
Step 2: Select your USB drive from the Storage Device list and click Search for Lost Data.
Step 3: Preview the search results and select the files you need and click Recover to get them back.
After successfully recovering data from a USB flash drive that's not recognized by Mac, it is time to format the USB drive on Mac. After the reformatting process is finished the USB flash drive should be recognized by macOS and show up in Mac Finder correctly.