Hi.
You should provide more details like:
1) is it a RAID? If yes, which one?
2) which kind of disk do you use? Sata / eSata / USB AND 7200 RPM / 10000 / 15000
IMHO this is a normal behaviour because of the troughput of your disk. You can open the RESOURCE MONITOR and see how much (in %) is your disk busy. If you see 100% and 50 MB/s then your disk cannot do anything more, change your disk with a faster one.
Let me know.