linux - Unable to mount hard drives in Debian installer shell

I am trying to mount a laptop hard drive during a Debian install for the purpose of backing it up to a USB external drive. I can't use the external drive from the existing OS on the laptop because it is so old it cannot read the external drive. When I boot the machine from the Debian CD I interrupt the install process at the "provide host name" screen, then select the run shell option from the menu. The installer announces I will be using the ash shell from a ramdisk and that the local hard drive partitions have been mounted to /target. fdisk confirms both the presence of the hard drive partitions (sda1-sda7) and the external drive partition.

Unfortunately I can neither find /target nor can I mount the drives manually. There is no /target directory on the ram disk.

If I try to mount any of the partitions using a command such as:

mkdir /mnt/sda3
mount /dev/sda3 /mnt/sda3

I get the error "invalid argument" and the dmesg is a bread (block read) error on sda3. I have also tried different -t (type) options with the mount, such as ext3, fat32 or vfat (for the FAT partitions), nothing works, same error.

If this error is because the partition type must be specified how can I find out the authoritative label to be used with the -t option? fdisk -l does not provide this label.

MORE INFO

The fstab is kind of like this:

none   /devpts   devpts   default
none   /run      tmpfs    default
none   /proc     proc     auto
none   /sys      sysfs    default

Executing df gives the following result:

none       /run
tmpfs      /dev
/dev/sr0   /cdrom