Hello, We have found a critical issue in Mac OS X 10.6.5-10.6.6 memory management that could lead to Parallels Desktop virtual machine instability and even virtual machines corruptions in some configuration. We have filed the bug to Apple and helping them with investigation. Problem affects Parallels Desktop 6 Builds 11990 and 11992 running on Mac OS X 10.6.5 or Mac OS X 10.6.6 computers with more than 4CPUs and more than 4GB RAM, or with virtual machines running on external drives (USB or LAN storages). However, this problem doesn't affect virtual machines running on local hard drives of all MacMini, MacBook, MacBookPro and MacAir models. Until the fix becomes available please use workaround described in the following KB article: http://kb.parallels.com/en/9697 Thank you. UPDATE From now you could update to new build with fix included. http://forum.parallels.com/showthread.php?t=108763 Note: You could remove system flag from your VMs after update.