Compact Flash Card "The disk you inserted was not readable by this computer."

Just bought 2 new CF cards - SanDisk Extreme Pro 16Gb cards.

having completed the first job with them this morning, my MacBook Air is telling me

Disk Utility....

The disk you inserted was not readable by this computer...

It's not the card reader i'm using that is causing the problem, as other cards are working OK via it... the cards in question are also working fine on my other laptop - a PC and have recorded photos this morning without any problems on my Canon 1D's

I need a fix for this ASAP.

PLEASE!