...... Here is what's up ....
OS X is basically a modified version of BSD so .... it sounds like everything is working as expected. Here are some things to pay attention to:
1. Remember that most drives usually transfer data around the 4MB - 10MB per second range. When you tell the OS to copy a file to another location, the OS will indicate that the file has been copied BEFORE it is actually PHYSICALLY done. What you want to do is open a terminal session (command line) and type "sync" (leave the quotes off) and then enter. This will tell the OS to write ALL buffered data to the physical drives immediately. The prompt will return when all data has actually been written to the disks. At this point it is generally safe to unmouunt the disk.
2. The OS also, in most cases, is writing or updating the file system journal periodically as long as the drive is mounted. Again, doing a sync will force this activity this to complete.
3. Most modern disks have write caches built in. sync will cause the caches in both the OS (FSCACHE) and the disk to be writen (FSFLUSHED) immeadiately to the disks and it will close any open file handles. If the file is still in use, the OS will subsequently reopen the file as needed until the application closes the file (like when you are copying files).