2. I formatted it with the inbuilt file manager, so whatever the default format is. So far I have copied files up to 2.5gb on to it
sudo fdisk -l
3. Using the N900 as mass storage via windows 7, I Get 13.2mbps transfer speed (from a sony vaio z with ssd drive and usb 3)
sudo hdparm -tT /dev/mmcblk1p1
Erm, default format would be exFat - N900 shouldn't be able to mount it at all, at least without special software support. Are You sure (again) that we're talking about 64GB SDXC card?...