$ fdisk -l
Note: sector size is 2048 (not 512)

Disk /dev/sdb: 30.0 GB, 30005821440 bytes
255 heads, 63 sectors/track, 912 cylinders
Units = cylinders of 16065 * 2048 = 32901120 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1           3       96264    0  Empty
/dev/sdb2              46         912    27856710    b  W95 FAT32
/dev/sdb3               4          45     1349460   83  Linux

Partition table entries are not in disk order
peteb@max-power:~/ipod$ sudo mkfs.ext3 /dev/sdb3
mke2fs 1.38 (30-Jun-2005)
Could not stat /dev/sdb3 --- No such file or directory

The device apparently does not exist; did you specify it correctly?