Time Machine once worked, now external hard drive is "read only"

Question

I have a 2005 iMac G5, running OS X 10.5. Had some power issues, and reinstalled (and archived) my OS a couple of time.
Recently purchased Western Digital 1TB external hard drive for back up. It worked once or twice, but now shows "read only" status.
Disk Utility won't do anything, because it's read only (can't repair, erase, partition or convert).
Hard drive works on PC running Windows 7, and newer Mac with Snow Leopard, just not the one computer I need it for. Any suggestions?

Answer 1

This may help:
What to do if Time Machine reports you don't have permission
This was posted in the Apple Discussions by V.K. I have only modified it slightly to be more generic.
The problem seems to be that 10.5.6 changes permissions on a file so that even the root user doesn't have write peivileges. I have no idea why they did this. The workaround suggested in [an]other post will probably work, too, but i did something less drastic. Instead of deleting the file I changed permissions on it, and it worked just fine. An added benefit is that the permission change seems to stick, so you don't have to delete the file every time you change a drive.
[Open the Terminal application in your Utilities folder. At the prompt [enter] the following command:
sudo chmod 644 /volumes/"TM drive name"/.xxxxxxxxxxxx
The name of the file .xxxxxxxxxxxx is based on the MAC address of your computer and will be different for every computer. Put the name of the TM drive in the above and keep the quotes.
You'll have to enter your admin password (which you won't see) which is normal.
This was edited by Kappy just for cleanup.
Enable Finder to Show Invisible Files and Folders
Open the Terminal application in your Utilities folder. At the prompt enter or paste the following command line then press RETURN.
defaults write com.apple.finder AppleShowAllFiles TRUE
To turn off the display of invisible files and folders enter or paste the following command line and press RETURN.
defaults write com.apple.finder AppleShowAllFiles FALSE
Alternatively you can use one of the numerous third-party utilities such as TinkerTool or ShowHideInvisibleFiles - VersionTracker or MacUpdate.
Also visit the User Tips found at the top of this forum's Topics list: Time Machine User Tips.