If you installed sp6 over sp3 you can uninstall it and it will go back to sp3 but if you've gone from sp4 or sp5 to sp6 you can go back only one step. Best of luck.
In the first place why would you go baxkward? If you install SP3 over SP6, you will more then likely be reformating. If the program is like all the other programs it will not allow you to do this any way. In which case you will be a very lucky person.
Is it a AST Manhattan Commerce Pro server? One of our clients has the same server which hang after upgraded from SP3 to SP6. I suggest go to www.ari-service.com to get the latest BIOS upgrade. After that, You can try the upgrading again. Out client has no problem with the server running SP5 at the moment.
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SP3 reinstall over SP6 ( AST Manhattan P