Well, I'm sorry but I can't just guess... can You verify that the partition hda11 exist? Does it have a file system? Can You mount it manually using

mount /dev/hda11 /mnt/tmp

(create the dir /mnt/tmp first)

I didnt create partition for hda11
I just add it to fstab
and try to mount it.
Is it necessary to create partition

Yes, otherwise You will have nothing mount. What You can mount is a partition with a filesystem on it.

fdisk -l

Disk /dev/hda :8455 MB

Device boot Start End Id System
/dev/hda1 1 126 83 linux
/dev/hda2 127 1027 83 linux
...
...
...
....
/dev/hda10 778 794 82 linux swap
/dev/hda11 795 1027 83 linux (I added this partition later)

Disk /dev/hdb:40 GB

Device boot Start end id system
/dev/hdb1 2 2550 f W95 EXT'd(LBA)
/dev/hdb2 2551 4111 7 HPFS/NTFS
/dev/hdb5 2 2550 7 HPFS/NTFS


cat /etc/fstab

/dev/hda10 swap swap defaults 0 0
/dev/hda11 /back reiserfs defaults 1 2
/dev/hda1 / reiserfs defaults 1 1
/dev/hda5 /usr reiserfs defaults 1 2
/dev/hda6 / home reiserfs defaults 1 2
/dev/hdb2 /dev/hdb2 ntfs ro 1 0
/dev/hdb5 /dev/hdb5 ntfs ro 1 0




I still cannot mount /dev/hda11??

???????

Your fstab says that the partition should have (is expected to have) a file system of type reiserfs, have You created that? If not, run the command

But You must be sure that it is the correct partition, if You run that command on another partition You may erase everything on it. Just wanted to tell You to be careful

After that You may mount it with
mount /back
or
mount /dev/hda11
or
mount /dev/hda11 /back
or even
mount -t reiserfs /dev/hda11 /back
and even more variants

The entry in fstab actually makes sure that it will be mounted every time Your system boot (thats when a mount -a will be done, usually), but right now it also helps specify the details, that explains why for example mount /dev/hda11 and mount /back work equally well.

Good luck!

/Lakris

thanks Lakris.
It worked.
One last question:

As I posted above, the output of "fdisk -l" shows
/dev/hda and /dev/hdb.

What is the diffrence hda and hdb?I didnt create hdb.
Can I remove hdb?

Sorry for any inconvinience