External Hardrive not detected

Apparently, you've somehow managed to accidentally disconnect the

externally enclosed hard drive without correctly unmounting it first...

Sometimes I've done this and can get it working again if I can get the

Disk Utility to 'see' the external drive. Other times, an enclosure can

fail while the drive within may still be functional; and if the issue is an

enclosure failure, a replacement could restore access to the drive.

But first see if Disk Utility can detect the drive; if the cable can be

replaced, test connections with another cable. And another port.

Here's a generic article about using Disk Utility to rediscover a drive

that cannot be seen by the Finder...

• External Hard Drive Unmounted or Invisible on Mac?

http://www.itninja.com/blog/view/external-hard-drive-unmounted-or-invisible-on-m ac-how-to-recover-files

And here's a search I recently made to show there are other similar ideas...

https://www.google.com/search?q=how+to+rediscover+external+hard+drive+once+improperly+unmounted

...having troubleshooted external enclosures and what-not, I've not used

these articles. Some enclosures have circuit boards inside that fail; so it

may be possible to obtain replacement or identical working parts to fix them.

Hopefully you can restore access to the drive + enclosure, for the content;

and be sure to make another backup archive on yet another device.

Good luck & happy trails! 🙂

Could be the unit has failed on another level, but usually you'd see

a standard notice on the screen if or when the external drive or device

had been accidentally unplugged. The warning (when apparent) says

something to the effect 'Disk Not Properly Ejected' or 'Unmounted'

and when this happens, the content on the drive may become damaged

or unreadable by the Mac.

If the cable can be replaced (both ends with plug connection) then you

may see if exchanging the cable can help. Also restart with the hard

drive attached. Sometimes this may let Disk Utility 'see' it, if not Finder.

If you have a PC or other non-Mac system, there are a few times where

another computer may be able to see these drives; but to use them is

yet unlikely because of the format difference. You can still lose content.

There had been a few good third party disk utilities that may help

recover content, however the drive would have to be located by

the macOS or another system. Some of these utilities have a 'free

scan' to see if their software (retail purchase) could locate/recover

the content of a hard disk drive or flash drive. Then, you pay a fee

to unlock the software. I've doubted their claims, to percentages.

Companies such as ProSoft Engineering have Data Rescue software

utilities that could be useful; some versions are available on USB flash.

•Data Recovery Software For Mac And PC - Data Rescue - Disk ...

https://www.prosofteng.com/data-rescue-recovery-software/

Recent search of current version w/ reviews: 'data rescue 4 review'

Another name that comes up, still a retail product, is Disk Warrior.

Each of these tends to have some specific angle they do best at.

•DiskWarrior 5 - The Disk Utility for Mac Disk Repair, Mac Directory ...

www.alsoft.com/DiskWarrior/

Others, such as 'Drive Genius' are usually for system boot drives,

so their usefulness may be limited even when used for a process

they were intended. See 'Prosoft Engineering' site details for this.

These usually only work if the drive can be detected on the bus,

or if swapped out to a known-good enclosure. Or, if you happen to

have an equivalent product to 'Universal Drive Adapter USB' from

newertech; as this example listed at otherworldcomputing online:

https://eshop.macsales.com/item/NewerTech/U3NVSPATA/

This can use a bare drive (includes its own power supply & cables)

to perform various tasks with hard drives, outside of enclosures.

Sorry to not have a 'method without instrumentality' as most these do

do require something or another - that most people may not have...

Or have and did not use, such as a method to create duplicates, &

extra drives on which to archive them, & no time before trouble set in.

Good luck in any event...! 🙂