Hi
It sounds like this is a single ESXi host.
You would have to copy/backup all the contents on the existing datastore to another location (external HDD) while all VMs are shut down.
Once that is done and you are sure the copied data is in tact you can:
- remove all VMs from inventory
- delete the datastore
- Shut down host
- replace HDD with new ones
- Start host and reconfigure new raid
- Create new datastore in ESXi
- Copy/restore all data back to the new datastore
- Browse datastore and add all VMs to inventory (create a list to make sure you get them all)
- Start VMs
Might be a good time to do a upgrade to ESXi5 as well 
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