The error message, "A general system error occurred: Failed to start fdm service on host," points directly to an issue with the vSphere High Availability agent, known as the Fault Domain Manager (FDM), on the specific ESXi host. When an administrator enables vSphere HA on a cluster, vCenter Server instructs each host to start its vmware-fdm service. If this service has been manually disabled in the ESXi host's configuration, the host's init system will prevent it from starting, resulting in the precise error observed. The problem is not with the cluster-level HA configuration but with the service's state on the individual host.

B. The vmware-fdm vib is missing from the ESX host.

If the VIB (vSphere Installation Bundle) were missing, the error would relate to a failure to install the HA agent, not a failure to start an existing service.

C. vSphere HA Admission Control settings are not configured correctly.

Admission Control policies manage cluster resources to ensure failover capacity. Incorrect settings would affect VM power-on or migration, not the startup of the FDM service itself.

D. vSphere HA startup policy is not configured correctly.

The VM startup policy determines the power-on order of virtual machines after a host failure. This is a post-failure configuration and does not impact the FDM agent's ability to start.

1. VMware vSphere 6.0 Documentation, vSphere Availability Guide:

Reference: Chapter 1, "vSphere High Availability," section "vSphere HA Architecture."

Content: This section explains that when vSphere HA is enabled, an agent (FDM) is installed and runs on each host in the cluster. This establishes that vmware-fdm is a service that must be running on the host for HA to function. A failure to start this service is a host-level problem.

2. VMware Knowledge Base, Article 2056299, "Troubleshooting vSphere High Availability (HA) issues in vSphere":

Reference: Section "The vSphere HA agent on a host has failed."

Content: This official troubleshooting guide provides commands to check the status of the FDM service (/etc/init.d/vmware-fdm status) and restart it. This confirms that vmware-fdm is a manageable service that can be in a stopped or failed state, and a failure to start it is a recognized troubleshooting step, distinct from installation or cluster configuration issues.

3. VMware vSphere 6.0 Documentation, vSphere Command-Line Interface Reference:

Reference: Section "esxcli system service," commands esxcli system service list and esxcli system service set.

Content: This documentation details how services on an ESXi host can be enabled or disabled. The vmware-fdm service can be explicitly set to enabled: false, which would prevent vCenter Server from starting it and cause the "Failed to start" error.