Iomega external hard drive either 'not found' by Time Machine or is now in 'read only' format

Question

I don't know what's going on with my iomega external hard drive.  Sometimes the HD is recognised by the computer; other times it isn't; if it isn't I can restore to an earlier back up via Time Machine app; but it's like the disk isn't writable.
Ten days ago, I had trouble backing up my iMac using Time Machine.  After turning it off and restarting my external hard drive, it backed up successfully.
However, today I am trying to back up my files, and TM tells me it can't find the external hard drive.  Nothing has changed -- I have been out of town this past week -- but somehow my HD now seems to be in 'read only' format.  It does turn on, and I could restore my computer to an earlier back up.
"Mac OS X can't repair the disk.  You can still open or copy files on the disk, but you can't save changes to files on the disk.  Back up the disk and reformat it as soon as you can."
Annoyingly, despite my HD not showing in Finder or on my Desktop (as it usually does), when I simply turn it off I get that warning message "The disk was not ejected properly".
Have tried verifying and repairing using Disk Utility, to no avail. 
Am prompted to reformat but I don't have a good understanding of what this means and how to do it.  I'm guessing this would wipe the disk clean and I'd have to create the initial back up image all over again -- I'm reluctant to do this since this HD is my only back up and if it all goes tits up I'll be up that famous creek without a paddle.
Thoughts, suggestions, input all welcome and deeply appreciated -- thank you!

Answer 1

fzgy wrote:
"Mac OS X can't repair the disk.  You can still open or copy files on the disk, but you can't save changes to files on the disk.  Back up the disk and reformat it as soon as you can."
It's possible a heavy-duty 3rd-party disk repair app can fix it, but they're expensive (DiskWarrior is about $100), and there's no guarantee it can do it.
Am prompted to reformat but I don't have a good understanding of what this means and how to do it.
That will erase it.  See Time Machine - Frequently Asked Question #5.
It sounds very much like the disk is beginning to fail, although it's possible there's a bad port, cable, connection or power supply (if it has its own).
I'd suggest getting a new one, and using it for your Time Machine backups; once you have a good backup there, reformat the old one (and select Security Options to write zeros to the whole drive -- if that fails, you know the drive is toast).  Use it for secondary backups, per FAQ #27.   If it has failed, get a second new one for secondary backups.