Hi,

there are several ways to mount a partition which is within a diskimage file created for example with dd.

The first and hard way  is to determine the offset of the partition start sector and mounting the disk by specifing the offset as parameter for the loop device.

You can use the  file or fdisk command to examine the file.
root@ubdev:/mnt/RaspberryPI # file 2012-07-15-wheezy-raspbian.img
2012-07-15-wheezy-raspbian.img: x86 boot sector; partition 1: ID=0xc, starthead 130, startsector 8192, 114688 sectors; partition 2: ID=0x83, starthead 165, startsector 122880, 3665920 sectors, code offset 0xb8
or

root@ubdev:/mnt/RaspberryPI # fdisk 2012-07-15-wheezy-raspbian.img

Command (m for help): p

Disk 2012-07-15-wheezy-raspbian.img: 1939 MB, 1939865600 bytes
255 heads, 63 sectors/track, 235 cylinders, total 3788800 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000714e9

Device Boot                               Start         End      Blocks   Id  System
2012-07-15-wheezy-raspbian.img1            8192      122879       57344    c  W95 FAT32 (LBA)
2012-07-15-wheezy-raspbian.img2          122880     3788799     1832960   83  Linux

For example the startsector of partition 2 is 122880,  this multiplied with the 512 bytes per sector is the byte offset of the partition. 122880*512=62914560.
Continue reading Linux: Multiple ways to mount a partition in a disk image file →