I guess you are not running udev, which would create the /dev node.
In that case you can make the dev entry yourself with
mknod /dev/sda b 8 0
where b is for block device and 8,0 are the major and minor numbers as shown in /proc/partitions.
It seems odd that there is no label on your pendrive. Normally there is an msdos label and you mount /dev/sda1.