Deadlocks can occur on a Windows host when attempting to access an iSCSI target running in a guest virtual machine with an iSCSI initiator, such as a Microsoft iSCSI Initiator, that is running on the host. This is caused by a flaw in the Windows cache manager component, and causes sluggish host system response for several minutes, followed by a "Delayed Write Failed" error message in the system tray or in a separate message window. The guest is blocked during that period and may show error messages or become unstable.

Setting the VBOX_DISABLE_HOST_DISK_CACHE environment variable to 1 enables a workaround for this problem until Microsoft addresses the issue. For example, open a command prompt window and start like this:

set VBOX_DISABLE_HOST_DISK_CACHE=1
VirtualBox

While this will decrease guest disk performance, especially writes, it does not affect the performance of other applications running on the host.