It sounds as if file permission are correctly but folders/directories may be wrong.
If so, then this should help.
You should check file permissions, owner ID and group ID for every folder that acts as a container for the location where you put the files on the target disk drive. This should have "system" as the owner and "admin" as the group -- this allows all users to access this folder.
The file permission ownership for folders inside the main folder should be set to each user ID. If you also need access the contents of these folders, then you may wish to set the group ID to your own account.
If you are using UNIX commands, then the "w" and "x" permissions should be true for directories. The "r" bit is set to allow the directory to be modified and clear to prevent files/folders from being added or deleted.
If none of this works acceptably, then you may find it useful to use the "Disk Utility" application to create a new disk image. You copy all the files into that disk image, unmount the image after it is full, copy the disk image to the target disk, set the owner as "system", set the group to "admin", and mark that one file as read-only. People may need to double click the file to mount it during boot or you may need to use "Automounter". This should allow people to see/read everything and change nothing.
