Just replacing the drives will not grow the array. When you replaced the drives it just rebuilt the data that was missing from the RAID array. I'm assuming you have it in RAID 5 configuration.
The device doesn't support growing of RAID arrays via the GUI. There is a possibility of using mdadm via telnet to grow your array.
I think this is how you do it, I haven't tried it myself. I'd highly recommend backing up your data before you try this.
Get the details on the current array
mdadm --detail /dev/md0
Check the device size is correct
Device Size : 1462766336 (1395.00 GiB 1497.87 GB)
Unmount the array
umount /dev/md0
Grow the array to maximum size, this could take some time.
mdadm /dev/md0 --grow --size=max
Now that the array has grown, the file system needs to be expanded to match.
resize2fs /dev/md0
Mount the expanded/grown array with it's resized filesystem
mount -t ext3 /dev/md0 /mnt/HD_a2
Check the size of the array
mdadm --detail /dev/md0
/dev/md0:
Version : 00.90.03
Creation Time : Sun May 17 14:37:06 2009
Raid Level : raid5
Array Size : 4388299008 (4185.01 GiB 4493.62 GB)
Device Size : 1462766336 (1395.00 GiB 1497.87 GB)
Raid Devices : 4
Total Devices : 4
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Mon Apr 5 15:41:43 2010
State : clean
Active Devices : 4
Working Devices : 4