It is possible that your hard drive had got some bad sectors. They may occur after some rough treatment. Try checking it for bad sectors with Windows scan disk or Norton Disk Doctor. If you would find any , then it means your hdd is dying and you'd better not use it for storing sensetive data.
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i tryed out the scan disk and it turned out nothing, said the drive was in perfect condition, then i did some deep scans with some other programs to check the disk out (Spinrite, Nortan Disk Doctor), it also said the disk was in perfect condition, so then i tryed somthing else, i put the HDD in the computer (it was in the External Case as mentioned above) and i tryed writing files to and a success, the files worked, i dunno how the External Case had writen the data all messed up (somthing 2 do with the firm ware perhaps?) any way i just wanted to post this to ask if anyone would know why (or how) the caseing had stuffed the Data transfer up all the time or anyone had heard of this befor?