It's hard to pinpoint the exact causes that render the flash drive not readable on Mac, but we will troubleshoot the problem and tell you how to effectively fix the unreadable USB drive on Mac: check if the USB stick is encrypted or has no file system, update the USB driver, repair corrupted USB drive on Mac with Disk Utility First Aid, repair external hard drive with Mac Terminal, recover files and reformat the corrupted USB drive, and find a USB repair technician near you.
For more details, please visit: How to Fix Unreadable USB Flash Drives on Mac without Data Loss