You got 2 rings without the heatsinks on because that's the default rrod.
If you have no heatsinks on, the 360 automatically shuts down with a 2 RRoD with a CPU/GPU overheating error code.
That's not your actual error code. Put thermal paste back on, put the heatsinks back on, and test for a RRoD. Then get the secondary error code and let me know what it is.
The center led flashing just means that the 360 drive isn't plugged in.
It won't RRoD because the drive isn't in, either.