Secondary volumes such as external USB and FireWire hard drives are often used with multiple computers, and if the drive contains a permissions-aware filesystem such as Apple's HFS+ (the default Mac filesystem) then it can inherit various permissions settings from the systems it's used with. While that's not usually an issue, these permissions can sometimes block access to parts of the hard drive so it can't be read from or written to.
To deal with this problem, Apple includes an option to ignore permissions on secondary drives, which can be invoked by selecting the drive and getting information on it, and then checking that option in the Sharing section.
While convenient, sometimes this setting may not stick. MacFixIt reader Ian recently wrote in about this issue:
Every day the permissions on the other 3 drives in my Mac Pro change away from "Ignore ownership on this volume." I can't access my other drives until I manually go into "Get Info" on each one!...
When you ignore permissions on secondary drives, the system does not remove or otherwise alter the permissions setup, but simply updates a central database to have the system handle the drive with root permissions. When the drive is attached, the system checks this database to see what settings should be used, but corruption in the database file may prevent the system from doing so properly, so that it resorts to the default setting of observing filesystem permissions.
To fix this, one simply needs to remove this database and have the system set it up again from scratch, similar to removing a corrupt preferences file when preferences aren't sticking for an application.
The permissions settings for attached volumes are stored in a basic text file called volinfo.database that contains a drive identifier string followed by a bitwise settings value to indicate various settings such as ownership and permissions management. This database is set up when you modify settings for a volume, but it's not needed for access to the drive, so you can remove it and the system will recreate it when you change the drive's permissions settings.
To do this, you will need to open the system's hidden database folder, by going to the Finder's Go menu and entering the following as the target folder:
When this folder opens, locate the file called volinfo.database and remove it, and then get information on your hard drives and set their permissions awareness accordingly.