Replying to this:

That strikes me as extremely uneconomical! E.g.: You can get a good quality external HD with a capacity of 1TB for the price of less than 2 Sandisk or Lexar 32Gb memory cards.

No it is not!
Quicker just to pop a new mem-card in then go though effort of making another backup!

You must have at least 3 copies, the working drive and 2 back ups.
Simply saving the card once full is a backup right here, and they are pretty small so easy to store somewhere.

Yes, yes, I did do what you said at one time, but with $10 for a 32GB SD card that is good for at least a year, it was more efficient backup wise just to just get a new SD card.

EDIT: Let me say, I do not take enough new photos to need lots of SD cards (otherwise I would do transfer & backup, then erase to reuse). SO I use the tiny SD card as one of my 3 backup mediums (after I DO copy the images to my other back-ups).