heavily used HDD's, simultaneous file processioning like that
defragmentation also helps to reduce file system errors
to fix this
go to properties of your drive & select tools->click check now in error checking & select the 2 check boxes that in pop up window..
its happened on 2 hard drives, they have other problems besides that, so i'm getting replacements, but i have 2 other hard drives that have had more use than these two, but to my knowlage never have an error once.
i found out the reasons why that crap was happening.
one was a bad sata cable. it has other problems too, but nothing severe. still got a replacement
the other one was on the verge of click of deathing, and it did, but i managed to get it to work long enough to data off of it.