I had an issue this week and have experienced this previously where the SCSI adapter of a virtual machine has been either created or modified to use the Paravirtualized SCSI adapter and upon power on the virtual machine may not boot to the operating system with the following error message;
Operating System not found
As I have experienced this issue in the past, I though it may be wise to put down what I did to resolve this issue for later reference, so here goes;
1) Power off the virtual machine
2) Select ‘Edit Settings’
3) Select ”Options’ tab, select ‘General’ and select ‘Configuration Parameters’.
4) Select ‘Add Row’
5) Enter the configuration parameter as ‘bios.hddOrder’ and the the value as ‘scsi0:0’.
6) Select ‘OK’ to apply the changes.
7) Power on the virtual machine.
More information may be found at the below knowledge base article;