HDD Disconnect automaticaâ??lly when format to be completed

Hi!... I want to ask you guys... My Mother's laptop, A-Note C-8226, use the WD1200BEVS 2,5" HDD, one day the OS Error when booting and must be re-installed, when i try to re-install the OS it says there's a problem... idk what... i forget... Then i dismount the HDD and try to install the OS from my sister's laptop... (the OS is windows 7 and i've found the way to install to USB HDD), the GUI to install to USB HDD says that i must have a partition with at least 15GB of free space, i make a 50GB partition for the HDD, then i format the partition, the HDD disconnect when the format to be completed, so i re-format it again... it do the same thing everytime the format to be completed... so... do you think... what is the problem?