Copying the entire HDD with dd

[Igor]

The machine has two identical hard drives.
One is the system disk, other one should be one-to-one
backup copy. I don't want to put my system on RAID1, thus
just want to completely mirror the disk - including not only files but the whole partition structure, boot sectors etc.
I supposed dd if=/dev/sda of=/dev/sdb bs=512 count=XXX,
where XXX is the disk size in sectors, would do the job,
but after copying disk this way I:
a) Do see the right partition structure with df (primary partition table copyed)
B) Is able to mount and access files on the first partition /dev/sdb1
c) Is UNABLE to mount any other partitions:
mount: /dev/sdb2 is not a valid block device

What's wrong?

[t.t]

Во-первых, зачем делать дубли? Во-вторых, сильно сомневаюсь, что человек по имени Igor не умеет писать по-русски.