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Also use Disk Utility to verify your host drive and the drive on which your VM sits, and turn off realtime disk optimisation if its on, also check...
Joel I recently upgraded to 10 and no problems so far, for some context, I use PD for Windows based software development and have around 6 VM's...
Windows 8.1 and Windows Server 2012 are certainly better, but ultimatley its the Windows applications themselves which have to support High DPI...
As serv says this is a problem with Windows software not correctly supporting High DPI scaling, as Retina or High DPI resolution displays are...
Hermann The 'Best for Retina' and other options for DPI are only available for Windows 7/8 and Windows Server 2012 Guest O/S types, for all other...