WD 1T stopped backing up says read only

Hi, I’m am not experinced with external hard drives, I have a WD 1T external hard drive around half full. I use it to back up my iMac.

It stopped working and a message comes up:

'Time machine could not complete the back up “My Book”.

Files cant be copied onto the back up disc because it appears to be read-only.

You may need to repair or reformat the disc using Disk Utility. If the disc cant be repaired, you must use a different disk for backups.’

So I went into disc utility but that brings up all sorts of options and this is where I get stuck as I dont know what any of them mean.

Any help would be very gratefully recieved - thank you

To partition and format the drive follow the steps on the following link:

How to partition and format a WD drive on Windows (8, 7, Vista, XP) and Mac OSX


Be aware that this will delete all data.

Make sure to backup all files if necessary.

This information is good if you have windows 8 as the drop down box works but all the other options don’t.

Once I knew there was no way of saving and reformating I moved all the back up files onto my computer then erased all the information on the disc, using disc utility to do that, in disc utility I then made a partition and renamed it then went into time machine preferances and clicked on the new name and started back up, it’s working well now.