if sda1 was about 50~100Mb - then that would be the w6/7 boot & system partition,
but since you say ~50Gb is missing, is that 750Gb the manufacturers disk size?.
coz - once it's formatted, then it is a little smaller - & M$ will 'see' a different disk size than a 'nix O/S would 'see' it as.
within the w7 install, be sure that it format ALL the available disk area - whatever the size it sees.
you may *not* get ALL of that 750Gb for usage under w6/7, anyway.
before you do that, however, use the 'nix live_cd to format the whole hdd to NTFS.
then boot that w7 DvD & install.
also check that the DvDs checksum is Aok & that the DvD was burnt properly.
Last edited by pierre2; 10-05-2011 at 06:00 AM.