The volume can't be ejected because it's currently in use
lsof is indeed your best bet. The fastest and easiest way would be this :-

sudo lsof /Volumes/myDrive


It can take a couple minutes to run, but once it's complete, it gives you a list of open files on the disk. The output will look something like this:

COMMAND    PID  USER   FD   TYPE DEVICE SIZE/OFF  NODE NAME
mds         89  root   19r   DIR   52,3      432     2 /Volumes/Photos
mds         89  root   23r   DIR   52,3      432     2 /Volumes/Photos
Finder     681 alans   14r   DIR   52,3      432     2 /Volumes/Photos
QuickLook 2158 alans    9r   REG   52,3  1141591 78651 /Volumes/Photos/_tmp_iphone_10_backup/APC_1546.JPG  


In this case, it's the QuickLook application that has a file open. Closing the application directly is the best way to fix the issue. However, that's not always possible. For example, QuickLook doesn't show up as an application you can get to in the Dock. 

If you can't close the application manually, you can use the kill command to terminate it from the command line. To do that, use the PID from the second column as the ID to kill. From the above example, it would be:

kill 2158


Note that sometimes that doesn't work and a more aggressive form of kill must be used. Here's a series of escalating aggressiveness (using the example PID of 2158):

kill 2158
sudo kill 2158
sudo kill -INT 2158
sudo kill -KILL 2158


You should be able to eject the disk once the process/application has been killed. 

One final note, lsof can take a minute or two. It can also hang, but you should give it at least a few minutes before you decide that's what happened. 

Also, sometimes the base command sudo lsof /Volumes/myDrive won't find anything. If that happens, try adding the +D argument (i.e. sudo lsof +D /Volumes/myDrive). That will do a top down scan of the disk. It'll take longer, but it should pick up anything that's causing the disk to be un-ejectable.

(Hat tip to Alec Jacobson's post for extra details.)