This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
There are a number of consultants who also write regularly updated in-depth reports (and sometimes post here) about various aspects of the mass storage market; if you're going to get into this business or are planning on spending many thousands or millions of dollars on equipment, talking to one of them might be a good idea. Sanjay Ranade (email@example.com) is one of the ones who both writes and posts here (he also has a couple of reasonably-priced books about mass storage). Infotech's reports include HSM, network backup, magtape and libraries. Others include Disk/Trend (Mountain View, CA, 405-961-6209) http://www.disktrend.com (good info there) and Freeman Reports (805-963-3853). Strategic Research Corporation has numerous white papers and good links available at http://www.sresearch.com, including networked storage. Some of them seem biased in particular directions, so caveat emptor.