Starting in September 2013, Windows Server 2012
R2 machines running as Hyper-V guests started issuing these warnings on reboot [8/18/2020: see comments—applies to 2012 but not R2]:
Log Name: System
Event ID: 32
The driver detected that the device \Device\Harddisk0\DR0 has its write cache enabled. Data corruption may occur.
Log Name: Directory Service
Event ID: 1539
Active Directory Domain Services could not disable the software-based disk write cache on the following hard disk.
Data might be lost during system failures.
It turns out this started after the patch described in KB2853952 was installed with the September 2013 updates. Before the patch, when a Hyper-V guest tried to disable write caching on a virtual IDE disk, it would report success even though it didn’t really succeed. With the patch, it reports failure.
The background on this is covered in this thread and this blog post [8/18/2020 link was no longer active; updated to refer to post on archive.org]. In summary: it’s a Good Thing that the guest knows write caching can’t be disabled, because then Active Directory falls back to non-cached updating. The 1539 message says “Data might be lost during system failures” but really it means “Active Directory data will NOT be lost during system failures because Active Directory will not use write caching.”