I was unable to copy the new NOOBS/jessie release on them (write error after 2-3 minutes).
I then downloaded h2testw, an sure enough, both cards failed once the process reached above 2048 Mb:
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Error creating file 'D:\3.h2w'. (The system cannot find the file specified. Code 2) Warning: Only 15270 of 15271 MByte tested. Writing speed: 7.49 MByte/s H2testw v1.4
just stay away from these readers: 1 reader was not working at all, and the next one was limited to 2048Mb
2X USB 2.0 TF card reader
So, for once, the SD card was no the culprit
Edit: RPDOM provided a logical explanation (post below)