Apple has changed the behavior of Time Machine in Mac OS 10.9.4.
When preparing a new drive with 10.9.4, Time Machine will change the attributes of the partition so that only the operating system and Time Machine can modify the drive (users are read only). See the example below:
This means that partitions / drives used for Time Machine are dedicated to Time Machine only.
When a user drags a file to a Time Machine partition or drive they will get a prompt saying "Modifying Drive X requires an administrator name and password" as shown in the example below:
Once the proper credentials are entered in, it will allow the copy to occur. Anything else will cancel the copy.