Rechecked "everything" and convinced cards are not genuine - they're certainly faulty in places. Rechecked "everything" again on Windows based machine (this one is Linux) and have arrived at same conclusion.
First suspicion (tell-tale sign) of faulty card was the progressive slowing of copying progress. Duplicated everything with different card readers / adapters etc to eliminate as much as possible before placing cards beneath magnifying glass.
As I type this (having driven across town to steal a 64Gb card out of my partner's Galaxy Note tablet) I am yet again copying files and will report upon the progress or lack thereof.
Checked path lengths, largest / longest was 253 characters - since shortened