There is no jumper or switch on Apple hard drives that make them read-only. However, there are ways to do this via software.
The first method requires that the hard drive be formatted by Apple HD SC Setup 2.0:
1) Start up from a floppy disk and run HD SC Setup 2.0 from that disk.
2) Select the hard disk that you want to make read only.
3) Click the "Partition" button.
4) A new dialog appears; click the "Custom" button.
5) Click on the Macintosh partition (the hard disk name appears in the partition).
6) A new dialog appears; click the "Lock" button.
7) Click the "Done" button.
8) Quit HD SC Setup.
The Macintosh partition of the selected hard disk is now write protected.
You can also use a disk editor, such as FEdit (this method works with hard disks formatted by earlier versions of HD SC Setup):
1) Open the hex representation of the volume that you want to lock.
2) Go to sector number 2. (This should be the third sector on the disk, with each sector having a length of 512 bytes.)
3) The sector should look something like this:
****
4244 9FAE F13C 9FAF 228B 0000 0010 0003
0000 063A 0000 0200 0000 0800 0004 0000
0020 0488 0855 6E74 6974 6C65 6400 0000
....... and so on,
where the 4244 begins at the zero byte of the sector.
4) The key location is where you see the word with the "****" over it. It should be located at the tenth byte and be $0000.
5) If the 8000 is at the tenth byte, change it to $8000 and write the sector to disk.
6) Restart the system to have the change take effect. (This information is usually stored in memory and won't be updated until you restart.)
Running software from a locked disk is not advised unless the locked device is NOT the startup disk. Many applications create scratch files in the System Folder of the startup disk. If it's locked, the software probably won't run. Some software even expects its home disk to be unlocked. If the locked device is not the startup disk, and the user is only going to read from it, then everything should be okay.