SOLVED: Mac Disk Utility suddenly won't recognize external hard drive - Help Please

SOLVED! For posterity for anyone who might Google this in the future, this is what works:

1. Open Terminal
2. Pull up diskutil list
3. Identify the disk you would like to unmount (for me it was disk2s2)
4. use the following 2 commands:
diskutil unmountDisk /dev/disk2s2
diskutil eject /dev/disk2s2

Eject was successful! Unplugged it, replugged it back in, and eureka! All of my files safely stored.


-----------original issue-----------
I was accessing my perfectly good, relatively new external hard drive on my Macbook Pro. Once I was finished with it, I clicked "eject" and the spinning wheel of death came up. Then, the whole computer locked up. All applications stopped responding, force quit worked but the spinning wheel of death remained and couldn't open any new programs. I couldn't even restart the computer. So, I manually shut it down with the power button. Didn't touch hard drive. Upon restarting my computer, the Mac won't recognize my external hard drive in Finder or in Disk Utility. I tried restarting a few times, which did not help. I unplugged it while the computer was off, and then restarted, and then plugged it back in, and that didn't help. I used a different connector, and that didn't help. Is there any chance of data recovery?

TIA. Ugh!

ETA:

Just tried this, to no avail:

1) open Terminal (in Applications > Utilities)
2) type in this command: diskutil list — this shows a list of all internal and external disks
3) locate your external drive in the list and find its identifier listed as: /dev/disk(#) where # is the disk’s number in your system
4) type in these commands to eject your external drive: diskutil unmountDisk /dev/disk# followed by the command: diskutil eject /dev/disk# make sure you replace # with the number of your external disk as listed in your system disk list

Errors:
Unmount of disk2 failed: at least one volume could not be unmounted
Volume timed out while waiting to eject