Random files corrupted & data lost

OS Windows 7 64bit.

I am using a My Passport 300GB USB Hard Drive to store my data as I work. To back up these files I use a My Passport Essential 250GB Ultra Portable USB Hard Drive with WD Smartware backup software and a My Book Studio Edition 1TB External Hard Drive using WD Anywhere backup software. I have recently noticed that a large amount of random files on the 300GB hard drive have been wiped of all data. The file still remains but shows 0 KB size, therefore impossible to open. I have checked my two backup locations to retrieve the files only to find that both backups do not contain these files, furthermore both backups are not up to date and incomplete. Can you advice on possible reasons in order to prevent any further data loss and any solutions to recover my very important business data?