How can I change 'Custom' permissions for TM backup disc?

Question

I have recently had some big problems with permissions on the main disk of my G5 iMac. After fixing permissions, Time Machine then reported that my backup disk (an external 300Gb FireWire disk) was read-only and so backups couldn't take place, and the icon for the disk has a padlock shown on it. Looking at the Get Info for the disk and unlocking 'Sharing and Permissions', I see a line saying 'You have custom access', and that all permissions shown are set to 'Custom'. I don't know what this means - and worse, I can't change the permissions to any of the normal settings (Read & Write, Read Only, Write only (drop box)), and I can't check 'Ignore ownership of this problem'.
I am thus unable to do any backup. The system worked fine for at least a year before this. Has anyone any idea what 'custom' means and what I should do. I haven't found anything that seems relevant by searching this forum.

Answer 1

Time Machine uses special "deny everybody everything" permissions. You should not change them.
FIrst, do a +Repair Disk+ (not permissions) on the TM volume via DIsk Utility (in your Applications/Utilities folder).
If that doesn't correct it,
If you only have a partial backup, or don't need the ones you've done, the simplest thing to do is just erase the disk/partition.
If you don't want to erase it, here's a workaround:
First, you need to find the name of the hidden file that's causing the problem Click here to download the +Time Machine Buddy+ widget.
It shows the messages from your logs for one TM backup run at a time, in a small window. If it shows a message like this: "You do not have appropriate access privileges to save file “.<nnnnnnn>” in folder <Name>"* skip to *Open the Terminal App* below.
If not, Click here to download the +Tinker Tool+ app. It allows you to change the Finder to show hidden files (among many other things). Select the first option under Finder, then click +Relaunch Finder+ at the bottom. Reverse this when done.
In a Finder window, select your Time Machine drive/partition. The very first file shown should have a name consisting of a period (dot) followed by 12 numbers and/or letters. (This is your Mac's Ethernet address). Copy or make a note of it.
Open the Terminal app (in your Applications/Utilities folder).
In Terminal, the prompt looks like this: user-xxxxxx:~ <your name>$
(where <your name> is your short user name). It's followed by a non-blinking block cursor (unless it's been changed via Terminal > Preferences).
At the prompt, type the following exactly as shown, substituting the name of your TM drive exactly, including any spaces, between the quotes; and the string of numbers & letters from the message where the series of x's are (keep the dot):
<pre> sudo chmod 644 /volumes/"TM drive name"/.xxxxxxxxxxxx</pre>
example: sudo chmod 644 /volumes/"TM Backups"/.0a1b2c3d4e5f
Press Return. You'll get some warnings and a request for your Administrator's password. Type it in (it won't be displayed) and press Return again.
Then try a +Back Up Now+ from the TM icon in your Menubar, or by control-clicking (right-clicking) the TM icon in your dock.