As far as common knowledge is concerned, see this thread
and scroll down to post #23379.
That post is irrelevant here. Failure to properly unmount a volume may cause data corruption; the post is talking about hardware damage. Data ≠ Hardware.
If the volume in question is journaled (as most are now-a-days), the risk of data corruption is much less than it was in the old days, but it's still not zero. You really want to properly unmount disk volumes.
One way to find out who's using a volume is to enter the following Terminal command:lsof | grep /Volumes/"Name of volume"
That will list every file that's currently in use on the volume, and which application is using it.
Some uses are benign. Finder almost always has some directory open on every volume, but will generally let go of it when it hears of an attempt to unmount the volume. Spotlight may be indexing the volume, but also will yield.
Some uses are temporary. An application may be updating something on the volume, and will let go of it once it's finished. Spotlight, as mentioned above, will stop indexing a volume that you try to unmount, but may want to finish the file it's on before doing so. In general, if you try to unmount something and are told that it's in use, wait a little bit (often as little as a few seconds), and try again.
Some uses are surprisingly persistent. If you use the cd
command in Terminal to make a particular directory the working directory, Terminal will not let the volume that directory is on unmount. All it's doing with the directory is focusing its attention there, but that's enough. To make it let go, either close the window or use cd
again to point to a directory on another volume. (cd
by itself, with no parameters, sets your working directory back to your home directory, which won't be on any volume you would want to unmount.)