Following instructions on various forums I formatted it to FAT 32 (it was originally exFAT32; whatever all that means).
Initially when I plugged the card through a card reader and usb connection to my Apple Laptop I was able to see that it contained 64Gbs, but because of its format I wasn't able to add any files to it. (I bought the card so that I could add various pdf books and reports to an ASUS tablet for ease of access and reference).
I formatted the device and plugged it back into the Apple and it now reads 32GB max. I have run a couple of data recovery software packages (iSoft and M3). None of them have found the missing 32Gbs.
The disk definitely did have 64 Gbs on it and it is a Samsung product purchased through Amazon from a supplier with good scores.