How can I write-protect (make read-only) a USB drive in OS X?
Option 1: Mount drive read-only

You can mount the USB drive read-only by using Diskutil.


First, insert/mount the drive once, run diskutil list from Terminal.app and take note of the device representing the drive (should be something like /dev/diskN with N being any number).
Eject the disk by running diskutil unmountDisk /dev/diskN replacing N by the number noted in the first step
Mount the disk read-only by running diskutil mountDisk readOnly /dev/diskN


This of course requires that you actively run these commands every time you want to use the archive stick. For an automated solution, have a look at MarcoPolo or write a launchd command (see e.g. here for inspiration).

Option 2: Use access control on the drive

Does the drive really need to be formatted in FAT32 or do you only access it from a Mac OSX system anyway? In the later case, you can save the drive content on another drive, reformat it as a Mac OSX drive, move/copy the content back and take away any write rights from anybody.