That varies. According to:http://elinux.org/RPi_Easy_SD_Card_Setup
The device is partitioned into 4 parts, a Fat32 (partition 1) a Ext4 (partition 2), a swap (partition 3) and an unallocated section.
Your attempted mount is for partition 1, which SHOULD have worked, IF the disk was initialized for the Raspberry PI. If it was not, then there might not be any partition table on the device, which would cause the failure.