NASLite Network Attached Storage

I wish to replace a hard drive which is faulty can anybody tell me do you have to unmout it in naslite then remove it, or can i just replace the drive ,What will happen when i replace the drive in the same space and mount again ? will naslite accept this ?

Working on the principle that it is a non-RAID drive, you would do as you have surmised:

Unmount drive;
Shutdown;
Remove Drive;
Install New Drive
Reboot;
Format New Drive (if not already ext2 / ext3);
Mount New Drive;
Reboot.

If you attach the drive to the same physical port on the motherboard / PCI card then it should appear in exactly the same place as the faulty drive was previously.

So, is there no way to unmount a (non RAID) drive, replace it and mount the new drive without turning off the server?

You'll have to reboot anyway so the drive is added to the shares.