When vSAN Data-at-Rest Encryption uses an external KMS, vCenter requests a Key Encryption Key (KEK) from the KMS and stores only the key ID, not the key itself. ESX hosts use the KEK to decrypt the Data Encryption Keys that protect the encrypted vSAN disk data. If a host reboots, it must retrieve the KEK from the KMS before its encrypted vSAN disk groups can be mounted. The guide states that when a host reboots, it does not mount its disk groups until it receives the KEK. Key persistence can allow continued operation when the key server is unavailable, but the documented persistence mechanism relies on TPM-backed key persistence across reboots. In this scenario, the host does not have a Trusted Platform Module and the KMS is temporarily unavailable. Therefore, the ESX host can boot, but the encrypted storage remains unavailable and unmounted until communication with the KMS is restored. Reference topics: vSAN Data-at-Rest Encryption, External KMS, Key Encryption Key, Encryption Key Persistence.