Unable to erase USB

Get Info in Finder shows the USB is now in "read" mode.

That will occur if there is a physical write protect switch on the drive, and it's locked in that position.

Assuming that's not the problem, I have found repeated attempts eventually succeed. The reason may be that the device is not responding fast enough, and Disk Utility is giving up too soon. Try a different format such as Mac OS Extended, then try FAT again. If that doesn't work try a different partition scheme such as MBR, and repeat.

I ran into as similar problem recently when I was also trying to create a bootable USB flash drive.

I managed to solve the issue by force unmounting the drive using diskutil from the command line.

I was following this guide:

http://www.amsys.co.uk/2014/05/disk-utility-tip-fix-couldnt-unmount-disk-errors/

After unmounting it I was able to erase and reformat as usual.

I haven't found any USB devices that didn't format after repeated and persistent attempts, but that doesn't mean that all of them are guaranteed to work. What model is yours?

These frustrating difficulties formatting SD and USB devices didn't occur in the past. Try booting macOS Recovery, or perhaps Internet Recovery (using ⌘⌥R), and using its version of Disk Utility to format it.

I haven't found any devices that became totally bricked. It wasn't necessary for me to use a different Mac, or to restore a Sierra system to make it work, or even to use diskutil in Terminal (although that's another option to consider) so keep trying.

Had exactly same issue. I kept repeating the formatting (Fat32) process and after 4 times it finally took.