Given your screenshot with the CPU usage gadget at 30% overall, with an i7-2600k, that is certainly not an idle PC situation. That may not be representative of the typical, low load CPU usage level on your PC, but if it is, you have all kinds of things running. Therefore, you'll have quite a bit of I/O in your file system to go along with that.
Windows 7 has many logs that are being written to all the time. If you've never seen them, from Control Panel select Administrative Tools, and then Event Viewer. Click on just about anything on the left side of that screen to view one of the many logs, and their many entries. Don't be scared immediately if you see some marked Error, check them and they'll likely be nothing.
So your previous SSD did not cause the HDD activity light to flash like the 520 does? You're not using a new case that has an extra bright HDD LED, or fixed the connection to that LED from the mother board, or something along those lines? Hmm, was your other SSD that screwy that the HDD LED would not come on when it was in use (or having issues?)
The drive activity LED on all my PCs are flashing all the time, I'd be worried if it slowed down or stopped, frankly.
