Easy solution use:
https://www.digitalrebellion.com/fcs_remover.htm
Here’s a recommendation from a reliable source:
https://www.lafcpug.org/reviews/review_fcs_maintennance_stone.html
But if you are not backed up and start clean when trouble hits you are asking for more trouble:
Get yourself an external drive and download CCC. Make a clone of your system. Now everything is backed up. Boot from the back up just to make sure everything went well. Repair permissions. Reformat your system drive and there you have it.
Since you only want to downgrade for one client, why not do a clean install of the OS on another external drive, install 5.x and run update? Then for this project only, boot from the external drive. You’ll have a clean system to work from. On the other hand, you might have trouble reinstalling 6.x on your system drive and will get the same advice to start clean.
The pro apps installs like to start clean. Sooner of later system drives develop problems and the best way to deal with it is with a clean install. Which means you need to have a complete back up and all your install disks of your apps. Almost all else are documents, which can be easily dragged over to your new documents folder from a back up.
Using disk utility, you can make disc images of all your install discs and save them on the root level of your cloned back-up. Mount them on the desk top of your clean new system drive and launch them from there. This is time saving because you are now installing from a hard drive not a cd. Plus no more loading and unloading cds. If you mount them on your desktop and get the first install disc going, it will automatically launch the others. Huge time saver.
Yes this will take a day. And you might need to tweak some things as you launch them such as preferences within an app. But you will also have clean and better functioning system. And when something bad happens like your system drive dies or gets corrupted, you won’t miss a beat. If your computer dies or is in for repair, you can now boot from your clone with a loaner and you won’t miss a beat.
If your stuff is not backed up now, you are asking for disaster.
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