I accidentally had Noise Reduction on Long exposure turned on in my Canon Rebel T3i. I took a 2 hours exposure and then the noise reduction kicked in and I was banned from using the camera for 2 hours (was not fun in the cold). I switched the camera off but that was held off by the noise reduction process. Is there a way to force it to cancel?
I had the exact same camera as you and experienced that too. There's no way to interrupt the process barring pulling the battery out. Even if you pull the memory card the camera will still try to save the image. Luckily the battery is easy to remove. Generally, the only photo to be corrupted is the one you were currently taking.
On a side note: try not to shoot exposures for that long, instead shot a bunch of shorter shots and stack them later. The reason for this is twofold:
- If something goes wrong on a two hour exposure that's the entire time to waste. If the exposure was for a shorter time then you wouldn't have to worry as much.
- While the camera sensor is capturing light it is heating up. The addition of heat like that will increase the sensor noise, resulting in a less desirable image.