You can quickly and easily increase the storage capacity of a RAID-5 array in Windows by using RAID-5's redundancy to sequentially add larger disks. Assuming that you have modern RAID controllers, firmware and management software, this works on most kinds of hardware. If your array has hot-swappable disks, it can be done without taking the server down.
Install the higher-capacity disks one by one and give the system time to rebuild the new disk between swaps. If there is a lot of data, this will take time. The time each rebuild takes varies depending on the amount of data on the disks and, more importantly, the characteristics of the RAID controller and software. Generally it will take at least 20 minutes per disk if the disks are mostly full. Once the last disk is installed and the system has reconstructed the data, you should see the new space available. Use Microsoft's diskpart utility to expand the Windows partitions to include the new space. You can either expand existing LUNs or add additional LUNs with the new space.
To use this technique under you should use either basic disks or dynamic disks with the latest service packs (at least SP3) installed.
It's important to note that while this method is fast, it does carry with it an increased risk of data loss. Needless to say, you want to have a current backup available, just in case.
For more information:
Advice: My first SAN
Advice: Optimize copying from a SAN to a regular RAID array
Advice: RAID-5 -- More reliable?
About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years, he has been a freelance writer specializing in storage and other computer issues.