@Gary
I know about formatting after downloading, but stll it's inconvenient when travelling for several days or weeks. Ideally, I would like to have a copy of all files on the card and one on the mac, in case something goes wrong with either. Now I just leave the images on the card until I get home. (I seem to remember as well that I got copying errors while trying to copy large amounts of data off the cards, so there still might be some compatibility issues with the card reader in the macbook air (2011 model).
@Richard
It,s exactly what happened to the defective card. Couldn't read it in Windows and Mac machines, couldn't even format it and none of the different image rescue applications could retrieve a single file. I don't remember if Disk Utility could do something with it, I'll pay closer attention next time (useful tip, thanks). I'm not sure though that these cards are formatted FAT, since they're too large for that - as I understood they're formatted as EXFAT or some variation of it, but I may be wrong.
(For the record, my cards are 64GB, not 128GB as I stated erroneously.)