[ Solution ] MacBook Pro won't recognize external hard drive

The problem faced by the user is as below :
I have a Seagate external HD with its own power source. I had previously formatted it extended journaled and turned on whole-disk encryption, but hadn't been using it. I wanted to add it as a second TimeMachine drive to my AirPort Extreme. I added a USB hub to the Extreme and plugged in the second drive, but it never showed up in Finder. (The first drive, also now plugged into the hub, was recognized and continues to work fine.)

So I plugged the second drive directly into my MacBook Pro. It showed up, I entered the encryption password, and then turned off the encryption. Finder showed no files on the drive, which I didn't think was correct, but couldn't remember for sure.

I ejected the drive and plugged it back into the hub plugged into the Extreme. It still didn't show up. So I again plugged it back into the MacBook Pro, and now it won't show up there either. When I plug the USB cable into the MacBook Pro, I can hear the drive spin up and the normal initial chatter, and its LED is on. So I believe the cable is good, and the drive is not broken in any gross way. I've shut down and restarted the MacBook Pro, which didn't solve the problem.

Does it show in Disk Utility?
Yes! It says the format is "Locked Logical Partion," unmounted. (The disk just has one partition, and the capacity shown is the full capacity of the disk.) When I click the "mount" button, it just blinks, and the new log entry says it is still a locked logical partition, unmounted.

Since at most the drive contained back-up copies of files I have safely backed up elsewhere, I decided I would just reformat it with a single pass erase. When I tried, I got the error, "Disk object invalid or unable to serialize." However, at the same time, I noticed a progress bar on the main Disk Utility window saying it was erasing, though the disk name it used (disk2s2, I think) was not one I had seen before. When I hit "ok" on the error window, the drive now appeared as mounted and showed up in Finder. DU says it has several folders and files, but they do not show up in Finder.

I ejected and plugged it back in, with no different results. So I reformatted again, this time with no erase, and it seemed to succeed, and the reported number of folders and files went down, but not to zero, and the used space only changed slightly. When I clicked on the drive itself, then back on the sole partition, the DU display went back to the original number of folders and files. Weird.
I was then able to plug it into my Extreme and everything seems to be working normally. 

Try the Following Solution

Triple-click anywhere in the line below on this page to select it:

{ diskutil list; echo; diskutil cs list; } | open -ef

Copy the selected text to the Clipboard by pressing the key combination command-C.

Launch the Terminal application in any of the following ways:
  • Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
  • In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
  • Open LaunchPad. Click Utilities, then Terminal in the icon grid.

Paste into the Terminal window by pressing the key combination command-V. I've tested these instructions only with the Safari web browser. If you use another browser, you may have to press the return key after pasting.

Wait for a new line ending in a dollar sign (“$”) to appear.

A TextEdit window will open with the output of the command. If the command produced no output, the window will be empty. Post the contents of the TextEdit window (not the Terminal window), if any - the text, please, not a screenshot. The title of the window doesn't matter, and you don't need to post that.

If any personal information appears in the output, anonymize before posting, but don’t remove the context.

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