The icon for Macintosh HD (my boot disk) shows as a Time Machine disk. There is a file .com.apple.timemachine.supported in the root of the disk. If I delete the file and restart the computer, the icon goes back to a normal HD icon. However, the .com.apple.timemachine.supported file is recreated at some point on boot because when I log in again, the file has been recreated. If then reboot again, the icon goes back to being a Time Machine one.

Any ideas about what is creating this file and why? More importantly - how can I get it to stop? It looks like something thinks the boot disk should be a Time Machine volume, but what?

Console.app shows the following messages at approximately hourly intervals:

19/01/2010 19:23:54 /System/Library/CoreServices/backupd[7459] Starting standard backup 
19/01/2010 19:23:54 /System/Library/CoreServices/backupd[7459] Cookie file is not readable or does not exist at path: /.<12 hex digits of MAC address for en0> 
19/01/2010 19:23:54 /System/Library/CoreServices/backupd[7459] Volume at path / does not appear to be the correct backup volume for this computer.  (Cookies do not match) 
19/01/2010 19:23:59 /System/Library/CoreServices/backupd[7459] Backup failed with error: 18 

Other possibly relevant information:

1. The boot HD isn't the original - the original failed so this is a SuperDuper'd clone of the original drive.
2. I used to use the same disk for a SuperDuper clone as for Time Machine.
3. These are the same same symptoms as this and this.