This depends exclusively on the RAID controller you use, but the answer is most likely that this wouldn't be possible.

What Raid Controller are you using?

I will be using NEC's own New powerful RAID controller (128/256MB cache, RAID 0,1,5,6) with optional BBU.How do you say that RAID controller decides this ?

IF you are talking about adding the E drive to the C drive, the C drive would most likely need to be configured as a RAID volume and then when the E drive HDD is available perform a RAID capacity/migration.

It is possible, dependent on the RAID controller type, if the C drive is already configured for RAID.  

I will be using NEC's own  RAID controller (128/256MB cache, RAID 0,1,5,6) with optional BBU.How do you say that RAID controller decides this ?

The RAID Controller doesn't decide the RAID level, that is configured by the user.  

I believe that it would be possible (it really depends on RAID controller).

But, why don't you:

1) Install and configure the server with two disks as RAID1
2) Make the RAID disk to be C drive. Install only the operating system on C
3) Once the SAN was configured, put all the Data to SAN disk.

According to the NEC Express 5800 Series Documentation:
http://www.necam.com/Servers/FT/320Fd.cfm

The Server will support RAID 1, I am not seeing 0,1,5,6.  Can you be more specific on the RAID controller?

Ok reading through the NEC 5800 Express 120Rh-1 user guide:

IMPORTANT: To use RAID 5 or RAID 6, with Internal RAID Controller (equivalent to N8103-116), you need to additionally install
the optional N8103-119 RAID Upgrade Kit.

On page 4-50 of the user Guide:
Reconstruction
The reconstruction feature is used to change configuration and/or RAID level of existing virtual disk. The Reconstruction contains the following three features, however, the Internal RAID Controller (equivalent to N8103-116) supports "Migration with addition" only.

IMPORTANT: You can use WebBIOS for Reconstruction. Universal RAID Utility does not support Reconstruction.

SO RAID Reconstruction is supported on this system, so configuring the C drive as a Single Drive RAID 0 and then later using the E HDD for Capacity Expansion through addition is supported:

User Guides:
Part1: http://support.necam.com/Servers/Rack/120Rh1/120Rh-1_UsersGuide-1.pdf (Raid Info found here)
Part2: http://support.necam.com/Servers/Rack/120Rh1/120Rh-1_UsersGuide-2.pdf

Please find the attached datasheet of the Server i am going to use.
SAN delivery was delayed and hence this makeshift arrangement, Please let me know
What are all the different options for me to handle this situation.

5800--120Rj-2.pdf

Thanks guys for all the quick response and suggestions......

I dont think my C will be at risk, because we backing up the complete file systems using HP DataProtector...Even if the C crashes i will be able to restore the drive with previous day backup.
Even i would love to add a 3rd drive for Data to avoid this overhead....

Any kind of step by step procedure with screeshot document is availalbe to do this.

Did you look at the manuals we both posted?  They have screen shots in them.

The manuals we provided have the step by steps instructions in them

Thanks. I am going to check the manuals and will get back to you soon for any queries.

Guys...very sorry for the delay...i am doing this on monday and will assign the points once completed....Thanks very much

Thanks guys..it worked