Activity Monitor, to test the read and write speeds of a USB flash drive. Open Activity Monitor and navigate to the Disk tab to view disk activity. At the bottom of the window, click the Data button to display the read and write speeds.
To test your USB flash drive, copy a large file from your Mac to the USB drive and back to your Mac. While the transfer is in progress, the read and write speeds will be displayed in the Activity Monitor.
Use iBoysoft DiskGeeker for Mac before to test the read and write speeds of external hard drives, and it’s an excellent tool. Additionally, it can detect more drive information and display it under the drive name, including health status, temperature, and more.
https://iboysoft.com/disk-utility/