Many times when there is a consistent flashing pattern on hard drive LEDs, it means the drive is in a failure state, especially when they won't mount. Most of the time there is either a clicking, or a very faint buzz/beeping sound. Sometimes, you can remove the hard drive and install it in another enclosure, as it is just the board inside the enclosure that has failed.
But, most of the time, it is the drive mechanism itself. It would be hard to diagnose further without being able to handle it in person. Are you able to see the drive in Disk Utility? Do the iMac's USB ports work with other devices? It is possible that there may be an issue with physical damage on either your usb port on the Mac, on the drive itself, or with the cable. You should replace the cable & try another port.
Updated September 30, 2018. You can use Disk Utility's First Aid on your Mac's startup drive. Is the need to boot from another volume instead of your normal startup drive. As an example, you may have a Western Digital drive listed, with two volumes. Failure,” this indicates First Aid failed at making the needed repair.
If the connection issue continues, then there may be physical damage with the port. If you think your drive is corrupted, and you have a backup of your data, then you can reformat the drive to wipe out the corruption and reuse the drive. If you do not have a backup of your data, then you will need to do data recovery. I'm a real novice but I have a similar issue with my WD Passport 1TB external drive. It all started when I upgraded my OS to OS X Yosemite 10.10.5.
The WD drive stopped backing up and became invisible to the Finder. I tried updating the WD firmware but that didn't help so I called support at WD and they told me the software is not compatible with the OS X Yosemite OS. They offer no further support or possible solutions other than buy a new backup. Don't think I want to buy from them since they don't support their device beyond 8 years. I know that is a couple of lifetimes in computers, but it doesn't seem long enough for a backup drive.
Any help I can get for retrieving my data would be helpful, there's only one port on the back. What is a good system to replace this useless piece of junk with? Sorry about your issues, jector1729. And what a lousy attitude the 'support staff' at WD have!
I will try and answer your questions as best as I can. First, the easy one: the best brands of hard drives (either traditional HDDs or SSDs) that 'play nicely with Macs' are Seagate for HDDs, and Samsung for SSDs. When I was using traditional HDDs, I always stuck with ones from Seagate, and now that I use only SSDs, I have all Samsung ones. They have always worked well with any Mac I have owned. Of course, it is best to take care of such drives (even internal ones) from a software perspective by doing routine/preventative maintenance (just like one does with an automobile). Myself, I am a 'stickler' for doing disk cleanup/maintenance/repairs from a software perspective. You can do a lot of disk cleanup on your own, and there are some excellent programs available (both free and commercial) that can also help with those tasks.
As for trying to 'rescue' the stuff off the drive, I assume the one (and only) port on the device is USB. I am not an 'expert' in this area, but here are the results of a google search for 'Data Rescue software for Macs': One of those links is for a product called (appropriately) Data Rescue 5: I have no direct experience with it, but a number of folks here have recommended it previously, and you can use it in demo mode. In actuality, assuming it works, it could be a real good investment to purchase it. Good luck, and if you can, let us know how it goes.
Thanks very much for all the information and the quick response, Honestone. Now I will tell you a very strange story: I've been out for the evening and I just returned home. I go to my Mac and it seems to be running very slow, so I do what I always do when it runs slow - I reboot it. When it comes back on there is a dialog box that says something like DO YOU WANT TO USE BACKUP DISC FOR TIME MACHINE? Now the WD backup disc had been invisible to the Mac since early October. Today I ran the Disc Utility, just trying anything in hopes the Backup drive would work, like that could happen without the proper software, right?
So I click on the YES button. The box closes so I bring up the Time Machine and sure enough, there is TODAY on top of the other pages!
So I go to FINDER and sure enough, my Backup Drive is listed under DEVICES and the circular arrows are running indicating that the Backup Drive is copying the designated files! How cool/Weird is that? So now I don't know what to think, but I am leaning toward: The WD guy lied to me when he said there was nothing I could do short of buying a new backup drive. Maybe the disc just needed to be run through the Disc Utility. Anyway, I am going to watch it for a few days and make sure it works and then tip-toe out of the room very quietly so the Mac thinks I am still there watching it. Any ideas are appreciated - this is very strange. I have so far good experience with WD hard drives on Mac.
But yes, I have heard of issues with the WD on macOS High Sierra. It’s strange that your drive have issues working with Yosemite. You may try to upgrade your Mac to a better macOS say Sierra or El Cap and try mounting your disk on it. Again, it’s a hit-try suggestion. Coming to the data recovery part; there is no free utility other than TestDisk, but you must know the command lines to run it.
I have never used it personally and that too for an external hard drive. Previously I had tried Stellar Phoenix on a SSD and it recovered most of the files. It was 256 GB and it failed after a while. You may also try Stellar but I recommend you read the features first But, with Stellar or Data Rescue you will have to pay the registration fees.
Actually, Yosemite was/is a quite stable Mac OS. It of course will not 'hurt' to upgrade to either El Capitan or Sierra (not sure how much 'better' either of them are, or even how much 'better' Sierra is than El Capitan), but if you do that, it would be prudent (and in some cases, necessary) to do the following: 1. Check of the compatibility of all your third party software and the OS you want to get to.
This might require downloading some updates. Do as much disk cleanup, maintenance, and repairs to your current 'system'. You can do a good amount of disk cleanup on your own, and there are some excellent programs available (both free and commercial) that can help you with that.
Next, make a backup to an external device. Time Machine is fine, but SuperDuper! Or Carbon Copy Cloner is better (plus there are a couple of issues with using Time Machine which I'll mention below).
The next step is to boot your machine in an 'isolated' manner. If you use Time Machine, the only way to that is to boot to the (hidden) Recovery HD partition. This link describes how to do that, and what you can do next: The first thing you should do then is to select that 4th option, Disk Utility, and have it 1) verify (and if needed) repair your internal drive, and 2) Erase and Format it. Once that is done, you would select the 2nd option, Reinstall macOS.
Here is where you'll encounter 'issues': First, I am unsure as to which Mac OS you'll get. Secondly, the installation will involve connecting to Apple's servers and performing the installation over the internet. Definitely not a speedy process. If instead you choose to use either SuperDuper! Or Carbon Copy Cloner in step 3 above, prior to that step, you would download the Mac OS you want from the App Store.
What will happen is a file entitled 'Install macOS El Capitan' or 'Install macOS Sierra' will wind up inside your Applications folder. When that is complete, make a copy of that file in another location. After you do the backup with either SuperDuper! Or Carbon Copy Cloner, it will be bootable. You would then restart your Mac from that backup.
Run Disk Utility from there to do the steps I described above, find and launch the file 'Install macOS El Capitan' or 'Install macOS Sierra' to perform a fresh, clean, 'virgin' installation of the OS onto your internal drive. That procedure has the following advantages: A. You have much more control. It is definitely faster, as you will be doing everything 'locally'. You know exactly which Mac OS you'll have at the end.
Finally, you'll be offered the opportunity, via Migration Assistant, to 'migrate'/copy needed 'stuff' from your backup. As for having to pay for a program that can rescue your data, my perspective would be that no price is too small for recovering such important and critical information.