That seems to have done it!

The Ubuntu box did not recognize the SDXC card in the reader (although the package said the reader was "SDXC ready").

A Windows XP box found the card in the reader, but offered only exFAT as a format option.

I downloaded Easeus PartitionMaster freeware from CNet to the XP box, & it had FAT32 as an option.

It formatted the card, & the N900 sees the card & seems to be happy with it. Only 59.4 G shows as available after FAT32 format on the 64 G card, but that seems to be the way it is.

Thank you pali, sixwheeledbeast, & TMO!