A *disk*?  Welll - part of the disk can be - in a way.
Think of a disk as just a bunch of unallocated space.

For example - you purchase a 300GB hard disk drive and when you first start
to use it, you take the 279.39GB of unallocated space and you create two
partitions on it (one 40GB C:\ and one 80GB D:\).  That leaves you with
159.39GB of space still unallocated - so you can later come back and
'partition' that unallocated space - if you desire.

Now - if you have already partitioned all the space and you cannot delete
the partitions that you created, then things get trickier and into 'special
cases'.  If - when you created the partitions - you made them 'dynamic
disks' - you *may* have a few options.  I don't recommend this for most
normal users.  If you just created them (or they were created for you) -
they are more than likely 'basic' disks and your choices all have to do with
third party tools.  Partition Magic comes to mind.