Unable to eject or erase USB drives

When you have this kind of error, typically it occurs because the device is mounted and there is something accessing the information on the mounted disk. Whatever is causing you to be unable to eject the disk is the same reason the disk utility can't unmount the drive to erase it.

Likely culprits for things still accessing your files include applications like Preview or possibly Dropbox if you use it to import the information. If the USB drives include songs, iTunes could possibly be accessing the information as well. You want to make sure that all of these types of apps are closed when you go to dismount.

You can use lsof to list open files:

sudo lsof|grep /Volumes/Volume\ name

If you run lsof without sudo, it doesn't include processes owned by other users.