After upgrading ESXi to 5.1 on an USB device, you may get a warning message stating that the system logs are stored on non-persistent storage.
Since version 5.1, system logs are kept on the default scrap partition. If it's not specified, it defaults to ramdisk, which is non-persistent; upon rebooting, the system logs will be cleared.
To resolve the issue, you can manually specify a location for the logs to be stored. This can be any configured datastore (VMFS or NFS). To configure this, go to the configuration of your ESXi host, Software, Advanced Settings.
In the tree on your left, find the Syslog, global configuration section and enter a datastore name in the field Syslog.global.logDir.
The location can be any configured datastore on your ESXi host. It's recommended to create a separate folder to store the logs. If you have more than one ESXi host, use separate subfolders as well. The change will be effective immediately. And upon rebooting, the logs will now be available on the datastore.