EX-FAT formatted external drive was working; now won't mount

OK - So I found the view All Processes pane in Activity Monitor and clicked on Disk where there is an entry in the list <<fsck_exfat>>... I believe you had directed me to Force Quit this ?

So with the entry selected, I can navigate down the View tab in Activity

Monitor and select <<Quit Process>>. (There's no option for Force Quit.)

Sorry to be such a stickler for precision, I just don't want to f*%k it up!!

It gives the message <<Are you sure you want to quit this process?

The process "fsck_exfat" is owned by root.>>

OK – duh! – there's the option for 'Force Quit'!!

Which I hit and got the following:

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macOS can't repair the disk "MyBook14TB"

You can still open or copy files on the disk, but you can’t save changes to files on the disk. Back up the disk and reformat it as soon as you can.

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I pressed OK and the futhermucker re-mounted magically on my desktop!

Hallelujah – the crops are saved!!!!!

(I owe you, Big Time.)

There is nothing in the Spotlight Privacy Pane and it's not available as a disk for me to add and then remove to jolt it into proper functioning... It does sound as though it might, indeed, be *indexing* on its own as it whirrs and emits muffled clicks intermittently. But it is obviously troubling that it won't mount. I wonder if I precipitated some kind of dyspepsia to its innards by plugging it successively into two machines running OS'es of such disparate eras...

So - newbie to the territory, here - I have never 'opened the hood' and poked around with Terminal. After starting it up my initial research suggested typing <<diskutil list>> then hitting enter. This indeed gave me a list of the drives which included the non-responsive WD MyBook14TB:

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/dev/disk2 (external, physical):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      GUID_partition_scheme                        *14.0 TB    disk2

   1:       Microsoft Basic Data MyBook14TB              14.0 TB    disk2s1

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When I typed <<All Processes>> it said command not found:

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Late-2011-15-MacBook-Pro:~ sethphineas$ All Processes

-bash: All: command not found

Late-2011-15-MacBook-Pro:~ sethphineas$

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Sorry to be so clueless, but I don't know even the baby-step basics of Terminal...