RAID controller, extended use of

I have used a RAID control card for over a year with no problems, nothing fancy http://www.newegg.com/Product/Produ...6115027&cm_re=hpt_2310-_-16-115-027-_-Product

My setup is RAID5 in a 3x1TB config, currently expanding to a 4x1TB array.
in my array i have a partition that is heavily used, and i had lost over 60GB from it.
I mean I have 840GB worth of files on the partition but the properties say 900GB. I verified my numbers to be accurate 2 different ways, so there was something telling my machine the partition was more full than it should be.

So this might have been the un-necessarily long fix, but since i had a fresh backup of the partition i formated the array, then replaced the data from backup and i recovered those lost GB's.
Anyone have an explanation where the GB's went? A way to avoid this in the future or better fix when it happens again?
Thanks