Windows Server 2003 Guest - Disk (d-drive) disappears

Hello all,
I have a Windows Server 2008 guest (64 bit) running on KVM. This is a guest which has been running reliably for several years now. There are 2 disk images, the C drive OS and a D drive for data.

I recently migrated the images from one storage environment to another. I also upgraded the host servers from Ubuntu 12.04LTS to 14.04LTS. I ended up recreating the VM template. The guest boots up fine, and runs fine for about a day. Then, for no apparent reason, the D drive just disappears. The guest is still running and accessible (c-drive is fine), but having the drive disappearing causes the applications to suffer obvious issues.

I have the disk images configured as HD prefix. Other than that. all settings are just defaults. Does anybody know if I should be specifying any particular options in the template? I’m thinking that maybe the newer kvm/libvirt maybe has different defaults settings that are causing the issue.

I am running OpenNebula 4.2.0 right now. If the answer is to upgrade ONE, that is fine, as that is on my radar anyway, but I think there should be setting to make it work because it ran as a guest on Ubuntu 12.04 previously.

Thanks in advance for any insight.