error code 43

I bought two SanDisk 16gb to make two backups for my imac.  The first one went fine and I unplugged it  I got an error message that the disk was not removed properly, which I ignored.  I plugged in the second Sandisk and tried to make an identical backup, but kept getting the “error code 43” message.  I plugged the first one in to make certain my backups were there, and they were fine.  Before I unplugged it, I right clicked and clicked on “Eject Disk”.  When I plugged the second one in and tried a backup again, it worked just fine.