Two things:
1) Upgrade to 10.8.3, it had a HUGE number of bugfixes.
2) The issue is highly likely to be the NTFS format on the external drive. OSX doesn't support it, and third-party drives are problematic at best. Reformat the drive as either FAT32 or HPFS+ (Mac format). I'd avoid exFAT at the moment too, as it's more prone to corruption. If you use FAT32, your VM's will need to be split into 2GB chunks.