disk stopped mounting, how to recover files?

Sorry for your frustrating problems. The bad news is that "occasionally clicking" is usually a sure sign of HD hardware failing.

The time to save your "year's worth of my work on this ONE external hd" was to backup as soon as you started having problems. It would be even better to have a routine backup routine for all important data to eliminate any single point failures.

Some details about your system might help:
What model/year Mac? Which OS? What make external HD and what was the format? What commands did try on Terminal? Do you have another HD available to copy files to?

Suggestions:
Do not try to do any further writing or repairs. You are right that you could get data corruption.
Try restarting your Mac with the external HD attached. If the HD mounts then be prepared to start copying your important files immediately.
Data Rescue has a demo mode to give you an idea if it can help, but it is not cheap.

 

Sometimes this helps (may or may not in your case):

1. Power down the Mac, all the way off.
2. Disconnect the problem drive.
3. Power up the Mac.
4. When you get to the finder, connect the external drive, and...
...wait. Just wait a while.

I've seen cases where (after connecting) the Mac will take some time to mount a problem drive. Give it 10-15 minutes before giving up.

If NOTHING else works, this might be worth a try:
1. Reinitialize the drive in Disk Utility (yes, you're reading that correctly), so it will mount on the desktop again (it will "look like" it's empty, with no files). DO NOT DO NOT choose to "secure erase". DO NOT DO NOT "zero out" the data. You just want a quick re-initialization that replaces the drive directory, and that alone.
2. Download "DataRescue" from ProSoft
3. Mount the "empty" drive on the desktop, and launch DR
4. DR will "ignore" the "empty" directory, and "go right to the platters" of the drive, scavenging around.
5. DR will present you with a list of files it thinks are recoverable.
6. DR will let you recover ONE File, and ONE FILE ONLY (this is to prove that it works before you pay the registration)
7. You now pay the registration fee, get a code, enter the code, and DR "goes to work" on the drive.
8. You will need ANOTHER DRIVE of sufficient capacity to serve as the "scratch drive" to receive the recovered files.
9. You will lose all previous folder hierarchies and probably lose file names as well. This is "par for the course" with data recovery.

No promises that this can work.
But it worked for me with a partition that disappeared and could no longer be mounted on the desktop.

Final thought:
If the drive has actual hardware damage (or controller damage), NO "data recovery" app is going to help you. The only way to get the data would be through professional data recovery, and that can be VERY expensive (we're talking thousands of dollars).