all dd copy do not work why whyyy?

all dd copy do not work why whyyy?

I try to copy one partition 500GB
sudo dcfldd if=/dev/sda12 of=/dev/sdb12
sudo dd if=/dev/sda12 of=/dev/sdb12
it do copy only 1GB it do it in to ram memory
after this /dev/sdb12 size is 1GB it is standard file not device
dd automatically is off and no count= is on in any command
all disks have no bad sectors smart is fully clean
md5sum /dev/sda and md5sum /dev/sdb is reading and thesame in
dcfldd if=/dev/sdb | md5sum
and all the time rebuildfstab script say can not stat device /dev/sdb
the sda 2TB disk is a boot disk
the sdb is also 2TB disk usb connected [in GUI after system start]
so all the same sizes the disk /dev/sdb is a raw copy of /dev/sda
did try to do this commands in frugal install linux:
xubunto=13.04
system rescuecd 2.8 and new one
parted magic 4.10
and others
in all linux cfdisk /dev/sdb show the same partition table
but gparted can not see /dev/sdb
and in sysrescuecd gparted can see blank 2TB /dev/sdb
no partitions I do not know why
all the changes is I did erase and encrypt one partition in /dev/sdb with
truecrypt 7.1a
why all linux can not stat /dev/sdb and what is it this stat ?