"ignore owner" disappears

[thomasx]

I am having a problem, that I am not able to figure out.

The Setting:
MacMini, 10.5.8, external HDD 1TB, File sharing enabled for some Macs locally, "ignore owner" is ticked in the apple-i-dialogue, so that every user has access to all files on this HDD, regardless who put them there.

SuperDuper is to SMartBackup those files, that have been altered, over the internet to a distant volume for offsite-backup.

every time the smartbackup runs, the "ignore owner" gets unticked. I have to tick it again each time, mostly when a local user informs me that he cannot access a file or write to a specific folder. he then has to disconnect/connect from/to the macmini again.

I have no idea, why this happens, but furthermore: HOW to STOP that from happening. I'd like to leave it ticked at all times.

Any ideas?