Is it one of these perhaps?
HP's current crop of <$400 machines.
It certainly shouldn't be an issue.
Is the Win8 machine running 64-bit windows? What about other machines - are they running 64-bit OS's, or 32-bit?
> Reads are in shared files, that's why there are locks.
Shared from where?
A busy LAN with large files is a different environment to your MacBook tests with smaller local files.
How many degrees of variability are there?
Can you test local (unshared) files on local disks for example?