Scotty,
What does it mean when the time machine says "read only volume  click time machine to change the volume".  It says that my backup failed.
I can't find my manual.

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It sounds like somewhere in your hard drive’s life its permissions changed. There is a way to bring it back to the state where it can accept data once again. This process sometimes involves using terminal which I don’t usually recommend people try. However, there may be an easier fix. You can try using the get info window.

Select the drive
choose “get info” from the file menu


Try to check “ignore ownership on this volume”
You can also click the plus sign and select yourself as a user then select to Read / Write then select the gear icon and choose to apply to all enclosed items. This may solve it but it’s likely there is a more serious issue at play.

Hope that helps a little.