This seems to have been a regular occurrence over my last two Macs and at least three OS versions (10.6/7/9, having skipped 10.8). Has usually manifested itself thus: the 'family' iTunes music library has its own partition on the internal HDD, and every so often one of the children will lose access to the library because the 'ignore ownership' checkbox has magically unticked itself. However, it appears to be getting worse. Three times in the last week my wife has been unable to launch Photoshop because the primary scratch disk (set to the above 'sound' partition) has been unavailable. Again, the 'ignore ownership' has become unchecked. Between the second and third launch failure I stripped out the ACLs from the partition, as described here. This seems to have worked, in that all files/folders now appear to have the same permissions (me=r/w; staff and everyone=r/o). I'm assuming that all this permissions are explicit, but that's coming from a position of knowing bugger-all about POSIX/ACL. Do implicit permissions even exist any more? New folders, and files saved directly to this partition take on the same permissions. The root permissions on the partition itself are r/w for each of system/admin/everyone; when 'ignore ownership' is checked they change to r/w for each of me/staff/everyone. Anyway... having done the above, Photoshop failed to launch for the third time and again this was corrected by rechecking 'ignore ownership'. Have subsequently given up and reverted to the startup partition as scratch. In the course of writing this I've had a few further thoughts, namely: a) Regardless of 'ignore ownership' settings, my wife's account should have sufficient privileges to access the root of the partition – she's an admin! she's a staff! she's an everyone! – which is *presumably* where Photoshop will do it's scratch magic, and b) Having turned on invisibles, Photoshop does not appear to create a temporary file on the scratch partition upon launch, so the hissy fit about scratch disk availability seems rather unnecessary. Hmmm. It seems like Photoshop might be bringing its own issues to the party, but it doesn't change my basic questions (congratulations if you've made it this far): 1) Do other people's non-OS partitions (whether internal or external) regularly lose their 'ignore ownership' setting? 2) If yes, is there a fix that prevents this from happening? 3) If no, am I doing something terribly wrong? 4) LATE-BREAKING OBSERVATION: a 'get info' on the partition suggests that I can make the entire partition a 'shared folder'. I can find next to no info on what a 'shared folder' actually does. Is there any magic to it, other than it changing the permissions of contents?