The Raspberry Pi is a great little device with literally hundreds of uses. And since it is so cheap, most devices that would otherwise cost a lot of money can now be built diy style on a very low budget. Especially for the Rasperry Pi newbie one very attractive project is to build your very own NAS. Most of the times you even have one ore more old external hard drives lying around, that just collect dust but you feel they should do more.
Most of the things you build with the Raspberry Pi, are surprisingly easy to setup with a very satisfying result. Just look at how easy it has become to setup a very powerful media center, that eats commercial settop boxes for breakfast regarding functionality and speed. More complicated projects, like a headless network-attached storage server, still need some knowledge to setup, and can become frustrating if they're almost working but not the way you'd like them to. Here I'm talking about having setup a headless NAS server to a level where you are able to just attach a USB thumb drive or hard disk and make it accessible to your network, but find out that the speed just sucks. Good news: it's very easy to fix.
Since the Raspberry Pi is not only cheap but also very (very) power efficient, some things work differently. Think of it like this: where your Desktop PC can draw as much power as it likes when doing things like copying files, drawing graphics etc this won't be a good idea on a mobile phone. The same happens on the Pi, also due to the fact that most of it components are very similar to what you find in a mobile phone.
In this case we have to deal with the design, that one very power efficient controller chip handles USB devices the ethernet network and more. That's why default Linux settings that wouldn't make a difference on a Desktop machine make a huge different on the Pi, and in this case it's the sync mount option. I won't annoy you with what this option actually does, just that it is always a good idea to not use it when dealing with usb devices you attach and detach.
If you let your usb device automount with "usbmount" you might have installed earlier, this is how to speed things up:
Your external device should now perform at least 5 times faster. On an RPI 1B this speeds up the download rate from 1MB/s to 5MB/s via samba.