First look at the partition table:
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#fdisk -l
then depending on what it shows /dev/hdb (or hdc--should be hdb though);
issue the command,
# fdisk /dev/hdb
select n to create new partition and then partition it as you wish.
Next;
Issue part probe command to refresh partition table in the memory
Then;
# fdisk -l ( see the differences)
format those partitions
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# mke2fs -j /dev/hdb1 ( and so on depending on the number of partitions )
Then you can apply a label on the partition if you want.
Next create any folder say /newhdd ( is on /dev/hda) and use it to mount point for the partions in the 2nd harddisk
#mount -t ext3 /dev/hdb1 /newhdd
Add an entry to /etc/fstab so that these partitions are mounted on boot.
Move the /home/<username> folder to /newhdd after your mount the partition.
Now create a symlink in /home dir of hda .
ln -s /newhdd/<username> /home/<username>