Change of Physical HD Designation in Disk Management

I have a Dell Studio XPS 9100 computer with Windows 7 Prof installed. The
computer has two physical hard drives that came with the computer. The
first physical HD had the following when it came from Dell: OEM partition
(39 mb), Recovery partition (10.8 gb), and OS (C) (71 gb).

The second physical hard drive was not partition or formatted when I
received the computer. I divided the second physical HD into three
partitions, Page File (D), Image Backup (E), and Data Backup (F).

When using Disk Management (computer management), in the lower pane, each
physical HD is designated as Disk 0, Disk 1, etc. In the Disk Management
window, To the right of the Disk 0 designation (a square block that has the
following information: Disk 0, Basic, 931 GB Online), will be listed the
specific volume(s) of that physical HD, which in my situation, is C:\ drive,
etc. My experience has been that the OS is associated with Disk 0 on a
normal setup, with just one OS.

However, when using my computer a few days ago, in Disk Management I noticed
that the Disk 0 designation was assigned to the second physical HD
(partition D, E, and F), and Disk 1 was assigned to the C: drive (OS),
Recovery partition, and OEM partition. After a few minutes, I rebooted the
computer, checked Disk Management again and the disk assignment was correct!
Does this mean that I have the first physical HD intermittently failing or
another type of intermittent hardware problem?

Any help would be very much appreciated as to why my computer changed
physical HD designations