I have a similar issue here:
Host:
ProxMox 2.3-13 (next step in our approach to solve the problem should be update of proxmox i think)
16TB HDD -> Hardware RAID 5
Guest:
MS Windows 2008 R2
HDD (ide0) local, raw 100GB C:
HDD (ide1) local, raw 100GB E:
HDD (ide3) local, qcow2 12TB R:
Guest is...