As said, I went by the photo store today, armed with my new K3 and an "almost new" multi card reader USB 2.0. I told this guy about the ideas exchange here at the Pentax Forum, and asked him to allow me do some tests on either one of the failed flucards.
This is what happened:
1) Both cards still showed the "memory card error" at camera start up. Camera got locked until turned off.
2) Attempts to format both cards on slot 1 were tried, but in both cases, again the "memory card error" showed upon pressing OK to format.
(From now on, we continued with only one card)
3) Flucard was inserted into built in card reader on a Toshiba Netbook computer running W7 32 bit (home edition). Card was not recognized.
4) On same computer, card was inserted to a Kodak A251 card reader (USB) and plugged into one of the Toshiba's USB ports. Again, even the little LED on the reader came on, the flucard was not recognized.
5) On another computer (Desktop) running W7 pro edition 64 bits, we inserted the card reader into a known USB 2.0 port. This time, the flucard was recognized. We formatted the card making sure it was under FAT 32 specs, but using fast format mode. After finishing, the flucard still showed the "memory card error" in camera.
6) We returned the card into the reader, but this time we performed a low level format (uncheck the fast format mode). It took, about 5-6 minutes to format fully.
7) After low level format was done, upon inserting flucard into slot 2 and turning on camera, a "card is not formatted" warning appeared. Turned off camera, moved flucard to slot 1 and formatted card in camera as usual. Then replaced the flucard back to slot 2.
IT WORKS!!!
Everything is back to normal. Flucard instantly established connection to my smartphone and everything went "by the book" from then on.
Conclusion: This behavior apparently is a known issue among users, but not documented properly neither in the K3's manual or the flucard's manual. The point is that if in any moment the "memory card error" shows, it means the card needs a low level formatting that is not possible in camera and needs a computer with the proper OS and hardware to do it.
Unfortunately, I cannot pinpoint or even say what are the minimum specs needed to perform a low level format on the flucard. I just found out a combination that worked among hardware at hand at this moment.
I also cannot say how long this situation will work or if the card will revert to error mode in some time after being stored away without use. In the meantime and until Pentax/Ricoh/Trek can figure out what is going on, I would advise to add this message as an instruction manual addendum for the flu card, and to the fine article written in this forum:
Warning:
If a "Memory Card Error" shows upon turning on camera,
remove flucard and perform a low level format in a computer
capable of doing so. Low level format is not possible in camera.
After doing a low level format, then perform a logical format in camera
using Slot 1 (K3) then return flucard to slot 2.
This should fix the problem.