With a standalone Windows Server, if a disk signature is altered, the OS might elect to mark a drive offline. Tasks such as changing a disk’s SCSI controller assignment or controller slot can change the disk signature, which could manifest in the drive going offline at the next boot. Before additional virtual disks are added to the VM, a PowerShell command can be issued to stop the OS from taking these drives offline. In an elevated PowerShell prompt, execute the following command to ensure that these drives are not marked offline during this change.
Set-StorageSetting -NewDiskPolicy OnlineAll
With a Failover Cluster, the default is OfflineShared. The reason is that in a cluster, when a new disk is presented, manually online the disk on one node, then add it to the cluster. From that point on the cluster will handle what has access to the disk.