I don't understand the instructions to fix my time machine.

My time machine fails way too often. Im using a 1TB toshiba external hard drive backup. The error always says it can't backup because the time mach. says "read only." I've tried to follow the instructions to repair for that error. I don't know what the instructions means by "mount" your external hard drive. and "your backup drive should "mount' in the disk utility sidebar when attached to your Mac. Sorry, but I'm tech deficient and figure it might just mean appear. However, I have repaired it by chance previous times but don't know how I did it. Now I got as far as the first aid/repair page...but it won't let me "repair." So I'm trying it again...first it has to verify, then when that's done it says "the volume time machine backkups appears to be OK." and doesn't allow me to "repair." So I clicked "restore" and I simply do NOT understand the instructions as it's talking about a disk image to a disk and destination. I can't get past this step. AND WHY does the Time Machine keep failing and going back to "read only?" Thank you for any help. (OK, I'm old and don't know what I'm doing) I still have Snow Leopard, osx 6. imac.