Find app that's using the disk

sudo fuser /Volumes/ExtDrive

will also give you the pid of processes with open files on the specified volume.

Howdy,

I'm not seeing a way to tell what's using the disk either with Activity Monitor or Disk Utility.

You've already done what's in If an app is using a disk you want to eject from Mac

Then there's If you can’t eject a disk from Mac

which seems pretty heavy handed but should work.

Have you ran Disk First Aid on the drive and it's okay?

ivan

1) When using the sudo kill -9, 
do I have to use cd to change to the disk 
or not necessary (I did use that, just in case, 
but I would like to be sure)? 
Does it kill those processes even if I'm not on the disk 
(in this case it's an external disk)?

No. Your process' current working directory has not affect on the kill command.

2) Is it possible that killing those processes will somehow affect the main disk, 
or are these process really just specific to that disk and killing them will have 
no effect on the main one? Hope it makes sense...

A process can have lots of files open, just one of which is on your external disk. Actually it will have files open on the boot disk, if nothing else the executable file, and various library and framework files.

The only downside to killing a process with open files, is if the file has not flushed all modified information from process memory to disk, then that information would be lost.

The integrity of the file and file systems is generally not affected.

Using the Terminal command line you can see what processes have open files on a particular volume. I suggest dragging & dropping the external volume onto the open Terminal window so it auto-completes the correct path after typing the following (make sure to leave at least one space at the end of the line after the "f"):

sudo  lsof  

For example if the external volume is called "ExtDrive", then the command will look something like this:

sudo  lsof  /Volumes/ExtDrive

You need to press the "Return" key at the end of the line to execute the command. You will also be prompted for your admin password, but nothing will display on the screen as you type the password. Press the "Return" key to submit your password.