Gartner and IDC have added up the disk storage systems numbers for the first quarter of 2008 and they both agree, that the market grew at around 10% in revenue year-over-year and at over 50% in capacity:
Total disk storage systems capacity shipped during the quarter reached 1642 PB (petabytes) according to IDC.
To give you a feeling about that number, check out Wikipedia!
On the other side, the "information producing" end of the industry, there was this study done by the University of Berkeley in 2000 and 2003 titled "How much Information". Now the UCSD (SanDiego) has initiated an effort to recreate that report for the end of our decade.
After all, Youtube, Facebook, as well as iTunes and other Web 2.0 applications did barely exist in 2003!