I don't think that will help... IDE virtual hard disks with Mountain Lion guests will encounter seemingly-random I/O errors and device/port/driver hangs, which is the reason for the OP's inability to manually mount the volumes, and is the reason we default to SCSI.
As far as I can tell, the most recent Mac systems with IDE/PATA hard drives were back in the PowerPC era (PATA optical drives were in Mac Pro systems as late as 2008), so it's not as if Apple is exactly exercising and maintaining this configuration. 
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Darius