The card needs formatting as FAT32.
They come as exFAT.

Use GParted on your desktop to reformat the card.
You can add swap spaces at the end of the card if you like.

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!

@rotoflex

My card has 57.80Gb but I have two 800Mb linux-swap spaces.
Your 59.4Gb sounds correct, the filesystem takes up some space, you'll never get exactly 64Gb

I just did it on-device when I first got it. root, reformat using cli, remove sd and re-insert.

would be great to get working exfat tools to avoid this for non-technical users.

A new problem!

I had copied the old SD card's files (& directories) to a drive attached to my Ubuntu machine.

After installing the new card in my N900, I connected the N900 via USB in Mass Storage mode to my Ubuntu machine, & copied the old SD card's files from the Ubuntu machine's drive to the N900's new card.

The subdirectories off of the root on the new N900 card are now apparently owned by the user logged in on the Ubuntu machine, & cannot be written to by root, user, all, or anyone on the N900.

I am only superficially Linux-agile. Is there some way in terminal on the N900 to change the permissions of the 8 directories off the root of the N900's SD card to rwe for all, recursively down through all the subdirectories?

Shows permissions.

chown = change owner
chgrp = change group
chmod = change mode

These will change permissions but learn how to use them first.

Arg.

In xterminal after 'root', command.
Top line is output of ls -la, showing permissions & owner of mp3 directory.

chmod is unsuccessful.

why dont' use sfdisk to set partition as FAT32 (type c) and use mkfs.vfat to format?

i did this and have absolutely no issue with it, in linux or windows

edit: And i did all of this on the device itself.

Another thing, why your windows will never offer you to format the card as fat32? It is because the format tool provided to format partition under windows does not support bigger than 32GB FAT32 partition.

Also, am i the only one that find weird to have access issue when using FAT32 partition? isn't it supposed to not use any kind of security attribute?

i mean, chown/mod/group on a FAT32 partition?

When you mount FAT32 in Linux, your mount options and /etc/fstab file determine who owns the files. You're right that chown/chmod/chgrp are meaningless for files on a FAT32 filesystem.

If the files on your card are RO, it likely means you need to umount and fsck the memory card. Once any filesystem errors are cleared up, you can mount it RW again.

Anecdotally I have much less trouble with ext2 microsd cards than I do with FAT32. Only occasionally, after a major system crash, do I have to fsck ext2-formatted cards... but with FAT32, it seems like they'll go RO for the tiniest reasons. Similarly, as long as ext2 FS'es are made with "-I 128", I find it easier to work with ext2 on Windows than I do FAT32 on Linux.