Your profile says you are using OSX Mountain Lion but you posted in iTunes for Windows. Which is it?
Are you trying to back up to a Windows formatted drive? You can't write to a NTFS formatted drive if you're running Mountain Lion. You will need to re-format the drive to Max Extended, erasing all the content in the process.
There could be other reasons for problems but we need more information on your system.
Thanks for trying to help me.
My IMAC is OSX Mountain Lion, have no idea what the hard drive is, or how to find out. I also have no idea how to reformat anything. I did save othe stuff from my old Mac, to that external hard drive in the past, so I assume it is correct.
so I need to know how to find out if that external hard drive is the correct one, as in your response and furthermore how to reformat it. I cant save them to thumb drives either, surely a thumb drive is universal.
This is why I hate Macs, they are not for IT non literates, whereas you get much easier to understand instructions on microsoft products and you can get someobne on the phone to talk you through problems. With the Mac, you ask a question and they send you a dozen links to read through which usually seem to confuse the matter further.
A prime example being my error above, posting to Itunes windows, I was smply following the links they sent me.
If you used the hard drive with another Mac then it is likely Mac formatted. It was because you posted in the iTunes for Windows forum (kind of the equivalent to taking a VW to a Ford dealership for service) I thought you might be using a Windows formatted drive.
Thumb drives are not universal. There isn't a "universal" drive format. Some work partly with both systems but are not fully compatible.
Click once on the hard drive icon on the desktop. Fo to File then Get Info. It will say the drive format.
Frankly I have never had your problem with a drive so I can't help too much except in general experience. The prohibitory sign is pretty universal and for some reason you are not allowed to copy to that drive.
I think if you had a Windows computer you'd be unhappy with it too and probably be posting on a forum that your machine was virus ridden and crashing and you didn't know anything about registries... To be honest, I was doing a presentation on a friend's Windows computer and wanted to get it to display a simple image file. There was no way we could get the .jpg image file to open with anything but Windows Explorer which is a web browser! I mean, that's really bad, and of course he was so used to using it without knowing anything about computers that he could not help me. I did my presentation in front of 30 people using Explorer.