Sale has ended but you can still get this app.
Download My SD LED Now

Official Description

My SD LED shows SD card (or internal memory) read/write activity in real time in the notification area like a real hard disk drive led.

Really useful if you wanna monitor your SD card activity or when you are copying a file.

Sometime your smartphone become unresponsive? With this app you can monitor if your system is reading or writing something. This is may the reason it is busy :)