Disable File System Cache for USB Devices?
On SuperUser Q&amp;A another user asked a similar question. The solution is a little bit tricky/efforty:

This should be possible by manually mounting the drive via the terminal. You'll have to eject the drive when you plug it in and then remount it in the terminal using the noasync option.
The best way to go about this would be to run mount in the terminal and take down the device so you know what to remount. Then umount /dev/disk1s1 (where disk1s1 is the device name) then mount -o noasync /dev/disk1s1. This method would also apply to most linux distros.
As you can see, it's easier to just remember to eject.