I used the command, "mount -t vfat -o wr /dev/sda1 /mnt/usb", and the removable usb harddisk was mounted.

The problem is, there are two FAT32 filesystem partitions and there EXT3 filesystem partitions in the usb harddisk, I want to put some files into the usb harddisk's certain partition, but I only can see one subdirectory that represent the whole usb harddisk in the "mnt" directory.

How can I put the files into the usb harddisk's certain partiton?
Thank you.

Try burning a CD with the files. Or you could use a floppy. Something similar to that could work well...

My trouble is not to install a distribution from the removable usb harddisk.
I mean that, for example, I want to save a file, xiang.txt into the second EXT3 partition of the removable usb harddisk, how can I confirm what is the exact second EXT3 partition?

Have you had a look at this site?

http://linux-sxs.org/hardware/flashreaders.html

Thank you.
The links you give is of very help to me.
Here is the my try:

[root@localhost root]# fdisk -l /dev/sda

Disk /dev/sda: 20.0 GB, 20020396032 bytes
255 heads, 63 sectors/track, 2434 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id System
/dev/sda1 * 1 394 3164773+ b Win95 FAT32
/dev/sda2 395 2433 16378267+ f Win95 Ext'd (LBA)
/dev/sda5 395 788 3164773+ b Win95 FAT32
/dev/sda6 789 1182 3164773+ 83 Linux
/dev/sda7 1183 1576 3164773+ 83 Linux
/dev/sda8 1577 2389 6530391 83 Linux
/dev/sda9 2390 2401 96358+ 83 Linux
/dev/sda10 2402 2433 257008+ 82 Linux swap

To mount second ext3 filesystem partition of the removable harddisk,
[root@localhost root]# mount -t ext3 /dev/sda7 /mnt/usb/debian_sda7

Thank you, and hope this is of help to someone.