Update:
After about 3 weeks. I plugged in again into my MacBook and left it there. After more than an hour I came back and the drive showed up on my Mac again. (So somehow the problem got fixed.)
I ran First Aid on Mac's Disk Utility. It says the drive appears to be OK. So I don't know what fixed it exactly.
Problem:
When I plug my WD Passport Ultra 4TB external hard drive WDBBKD0040BBK-0A (with 2TB of family photos) into my MacBook Pro it has fast flashing of light and can't be mounted. It can be read on a Windows laptop (after Windows automatically fixed an error). I tried opening up Disk Utility on my Mac, but it just spins forever trying to mount the external hard drive. Nothing ever shows up on Disk Utility.
How it broke:
The WD Passport Ultra was plugged into my MacBook on battery. I was not actively reading or writing to the external hard drive, but it did have a list of directories open (no individual photo files, but just the directory names). The battery ran out.
When I charged up my Mac again it says that the disk was not ejected properly. Ever since that my WD hard drive has the flashing problem and can't be mounted on my Mac. I tried changing the Mac laptop, changing the USB cord, same problem.
I then plugged in my WD hard drive to a Windows laptop. It says that it detected an error and had the option of automatically fixing it. I clicked "Start" and let it ran, but it didn't have a progress bar and just had a "Cancel" button. After running for 3-5 minutes, my mom came in and unplugged the external hard drive forcefully (I yelled at her for doing that). We tried plugging it into the Windows laptop again, and my files and photos could be read!! But it still can't be read on Mac and has the flashing problem.
Why is it like this? What is broken? How can I test what is broken?
I'm asking because I have had accidentally unplugged other WD external hard drives without ejecting in the past while not writing data, like in this case, but the drives never broke. What is up in this case?
I'm curious if this is something that happens a lot or if it's just a less durable hard drive I got. Because I'm at a point where WD's warranty can give me a new drive to replace it, or I can fix this drive myself. (I have copied the photos out to a new drive on a Windows machine.) Because if it is something that happens with the same frequency in all drives, shouldn't I just fix it?
What is the best free software program on Windows that might potentially fix my problem? Mac programs are good too if they can actually access it since my Mac can't even mount it.
Best Answer
This happens because OSX "forces" a disk check/repair in the background before mounting the drive (since it was ejected improperly), so it will simply appear to not be mounting until that completes. It seems it was fixed for you when you let the check complete--- A quick way to "fix" this is to manually force quit the fsck process from activity monitor.