Increasing the capacity of raid 5

My customer has a raid 5 array based on 4 x 300Gb drives, 1 of the drives is a hot fix, so the 3 live drives gives them 600Gb of usable space. The drives are a few years old and they need more space, so they've asked me to replace the disks with 4 x 1Tb drives. A couple of websites I've looked at indicated that this was a feasible proposition. The system is equiped with 4 hot swap drive bays. So I've replace each drive in turn, each time letting the array rebuild itself. Each time I replaced one of the drives the rebuild took between 20 - 22 hours. So now all the disks have been replaced and the array has finished rebuilding itself for the last time, but it still says the usable space is 600Gb. Disk management in windows doesn't show any additional unformatted space. So I rebooted the system, pressed ctrl-G and had a look at the RAID configuration, which confirms that while it can see 4 x 1Tb drives, the host drive size remains at 600Gb. There doesn't seem to be anyway to increase the size of the host size.
Perhaps someone here can confirm; the only way to increase the size of host drive is to delete the existing host (and lose all the data) and set up the whole array as new, then restore all the data from a backup. Hopefully someone can respond quite quickly, as I'm returning to the site in about 10 hours time, presumably with the bad news!Start Free Trial