What is the most likely cause of all hard disks in a server array failing?

The most likely cause of all hard disks in a server array failing is a backplane failure. The backplane is a crucial component in a server that connects multiple hard drives to the server's motherboard and allows communication between them. If the backplane fails, it can disrupt the power supply and data flow to all connected drives, potentially causing them to appear as though they have all failed at once. This failure can manifest as the drives not being detected by the system or not functioning properly, leading to a scenario where it seems that all disks have failed.

Other factors such as an incompatible BIOS, cache battery failure, or incorrect firmware version typically affect specific drives or components rather than causing universal failure across all disks in an array. While these issues can lead to performance problems or may prevent drives from operating optimally, they are less likely to cause simultaneous failure of all drives. Hence, the backplane's role in managing multiple disks makes its failure the most plausible reason for all disks in a server array to fail together.