My iMac suddenly can't read the backup hard drive I've been using for Time Machine. I did NOT just upgrade the OS or anything. The external HD is an OWC Mercury Elite All Pro. It's worked fine since I got the iMac 4 years ago.`

Question

My iMac suddenly can't read the backup hard drive I've been using for Time Machine.  I tried unplugging the cord that connects the HD to the iMac and plugging it back in, but I still get "The disk you inserted was not readable by this computer" below which are buttons for Initialize, Ignore and Eject.  I was using a cord that went from larger square plug to larger square plug.  So then I tried one that went from smaller square plug to what I think is USB (thin rectangular plug) of the sort that connects the keyboard and mouse. It's the type that my printers and scanners use to connect to the iMac.  I did NOT just upgrade the OS or anything.  The external HD is an OWC Mercury Elite All Pro. It's worked fine since I got the iMac 4 years ago. What else can I try before just trying to initialize and

Answer 1

Thanks, Michael!  I do hear it at times spooling up and running. Just after I bumped the thread I looked for troubleshooting for this drive online and found the manual which suggested using Disk Utility which I've seen before accidentally (if I hit Command Shift U instead of Shift U to type "Unit" on a new folder for a student's homework ) but had never really noticed.   Disk Utility does see it and also a sub-something (directory?) which might be the Time Machine archives on the disk, called disk1s2), sort of the way that my iMac's hard drive shows up as 640.14 GB Nitachi HDT7... and has a sub-something titled DB iMac, which is what I named my iMac's hard drive.
Anyway the owner's manual just shows the image under the formatting section, not the troubleshooting section, but as soon as I saw it in the manual I remembered seeing it accidentally a few times, went to it, and am now verifying the disk.  Right now it's telling me that it will take 2 hours to complete the verification, so I guess I have a bigt of a wait.  :-) 
Does that fact that Disk Utilities can see it mean it's not failed, or just that it hasn't completely failed? 
I can see the virtue in having multiple redundant backups, or at least two backups. What do you suggest?  Two external hard drives?  I had this one linked by ethernet, and but I also have a cord that could link it by USB (like a printer), so if this one is reparable I could get a second one and link it by USB.  If this one is not reparable I could get two and do the same thing.  I do have an Airport so I suppose it's possible to get some sort of Wi-Fi hard drive (my new printer/scanner uses only the network and not a cable, although it has a cable that I used for the initial installation), but I'd suspect a Wi-Fi hard drive might have a higher price.
What hard drives, if any, do you recommend? I seem to recall that when I was looking at external hard drives 4 years ago, Apple's were substantially more expensive, which is why I got the OWC Mercury Elite All Pro.