Sometimes your Mac doesn't like to let go of a disk and unmounting somehow fails.
Often this is because some applications is "locking" the disk and refuses to let it go for an unmount.
Now how do we find out what application this is?
First step; determine where the disk is mounted - in Terminal.
Quite often this is a directory in /Volumes/, for example /Volumes/MyDisk.
Next step we use lsof (list open files) in Terminal, but we do need sudo (admin) rights for that.
sudo lsof /Volumes/MyDisk
This should display a list, for example something like this:
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
mds 180 root 23r DIR 53,2 16384 2 /Volumes/MyDisk
mds 180 root 27r DIR 53,2 16384 2 /Volumes/MyDisk
Finder 11722 hans 51r DIR 53,2 16384 2 /Volumes/MyDisk
QuickLook 11732 has 9r REG 52,3 1143491 7862 /Volumes/MyDisk/SomePicture.JPG
Here we see that "mds" and "Finder" are currently accessing your disk.
In this care it's very unlikely that either of these "lock" your disk.
"QuickLook" on the other hand seem to have a file open - and is most likely the case the one locking your disk.
To stop this, you can "kill" that particular task as such - CAUTION when killing a task, unsaved data in the application will be lost!
First determine the PID, the Process ID. So here that would be "11732".
Next kill it:
sudo kill 11732