If a server shows an error message stating the operating system was not found after a new hard drive backplane installation, what should be checked first?

When a server displays an error message stating that the operating system was not found, it usually indicates that the system is unable to locate or access the OS files on the boot drive. After a new hard drive backplane installation, the first step should be to check the system board cables.

This is essential because the connections between the backplane and the motherboard are critical for the proper functioning of the newly installed hard drives. If the cables are not securely connected or if there are any issues with the connectors, the server may fail to recognize the hard drives or the operating system installed on them. Thus, ensuring that all system board cables are firmly connected and functioning properly is a necessary troubleshooting step.

Other actions, such as rebuilding the RAID array, reinstalling the server OS, or removing the new hardware, may become relevant later, but confirming the integrity of the system board connections is the most logical first step to address the immediate "operating system not found" error after modifications have been made to the server hardware.