Default [Q] Four questions about Nandroid backups

Hi,

Having CWM installed on the recovery partition, I tried to perform a Nandroid backup to the microSD card.
As I cannot find any valid partition system on the SDCard after the backup, I have the following questions :

1) Should the microSD card be formatted before making the Nandroid backup, and should it be in FAT16 or FAT32 ?

2) Which is the nature of a Nandroid backup : an ".img" file containing all partitions as well as the folders and files in them ?
The backup does not seem cloning the partitions but copying files from a mounted filesystem.
Is the backup some kind of archive of the copied files ?

3) At the beginning of the backup process, CWM tells "SD Card space free 6301 MB", whilst the microSD card is only 1 GB.
Does this mean that I need a 8 GB sd card to perform the full backup ?

4) To extract files from the backup using Windows or Linux, should I then use the "unyaffs" utitity ?

Thanks.