I've been using a Seagate external drive for backup since the last time. The e-books seem protected pretty well with their software "lock". The main downside is that it will only run on the drive and OS on which it was initially installed. When changing either of these, it seems that one needs to purchase updated passwords.

OK, a bit more than ten years out-of-date. But I think just about every collectors' book suffers the same fate to one degree or another, even the $300+ ones, as more is learned and documented. I view these as a concise overview of the topics covered and --like Wikipedia--a good place to start. I agree the prices are very reasonable.