every file on the disk should also follow suit automatically. How to do it depends on which version of the Mac OS you are running. You can find out by going to the apple in the upper right corner, click on that, and select "About this Macintosh" or in OSX, "About this Mac." The resulting window will show you the version of the OS that you are running. There is one procedure if you are running OS7, OS8 or OS9, but a different procedure if you are running OS 10 and above.
It's been a couple of years since I have used OS 8 or 9 so I can't quite remember, but I think you would click once on the disk then either go to the File Menu and select "Sharing" or select "Get Info." One or the other of those should allow you to check and uncheck boxes which share the disk and its contents. I can't really remember the exact procedure in OS 7, 8 or 9, but the answer is probably somewhere in those dialog boxes.
If it's OSX that you are running, you would click once on the disk, then go to the File menu and select "Get Info," then you will see a window with several triangles on the left, which expand the window if you click on them. Click on the one that says, "Ownership and permissions" and it will expand the window, the first item in the list should be, "You can" followed by a pop-up menu that probably says, "Read Only." Click on that menu and change it to "Read and Write." That might be enough to solve the problem.
If this still doesn't help, try going to some of the many Mac help web sites, such as:
http://www.macfixit.comhttp://www.macintouch.comhttp://discussions.info.apple.comhttp://www.maintoshos.comhttp://www.macsonly.comHope this helps a little.