First of all, there are several possible "busy" signals from the camera... For example, it can warn that autofocus is busy and unable to lock on, or it can indicate busy trying to write data to a memory card.
The usual reason for a "busy" camera is a slow memory card or trouble writing to the card, maybe because it's formatted incorrectly or there's "strange stuff" on the card that's interfering with new data being written to it. Pretty sure it's not the problem here because you say it only occurs with the one lens and, even then, only when both the diopter and the filter are installed. But, do you regularly format your CF cards in-camera? (Warning: formatting wipes the card of all current images, so be sure to download any you want to save, first.)
Next, it only happens when you stack a 500D close-up lens and a circular polarizer? Frankly I'm not a fan of diopter lenses (I use macro extension tubes instead) and I can't imagine there are a whole lot of situations that would require both a 500D and a C-Pol. But, all that aside... How are you stacking the two? Are you putting the C-pol on the lens first, then the 500D on top of that? Or the other way around... the 500D on the lens and the C-Pol in front of it? Have you tried both ways and do you get the "busy" warning both ways?
A polarizing filter has a pair or "foils" in it that have a pattern of openings and are sandwiched between glass layers, which block oblique rays of light, allowing only "aligned" rays of light to pass through. Circular polarizers have a different foil pattern than linear polarizers, in order to allow phase detection AF to work (I know of at least one case of a high quality B+W circular polarizer that was mismarked from new, and actually was a linear polarizer).
This is just a guess... but if you put a polarizer in front of a 500D, the close-up lens would then also magnify and change the size and relationship of the C-pol's foil patterns, which in turn may cause AF to struggle, fail and give you a busy signal... even though you're using "correct" circular polarizer. If I were using the two together on a lens, I'd install the C-pol first, then the 500D on top of it.
This is sort of like the guy who went to see a doctor with a complaint, "Doc, my shoulder really hurts when my arm is twisted this way behind my back." To which the doctor replies, "Well, don't twist your arm that way."
Maybe it's just not a good idea to use a C-Pol and a 500D together... Or they need to be used only in a certain order. A 500D doesn't change exposure much, lets most light through. A C-pol "costs" between one and two stops of exposure, depending upon how it's set. These, together with your ISO and shutter speed selections, and the lighting situations, whether or not your using a flash or adding light in other ways, may be forcing you to use longer exposure times.
The other possibility is what you mentioned: Long Exposure Noise Reduction (LENR). There are two types of noise reduction in the 7D. But the one to be concerned about is LENR, which is set up with Custom Function II, 1 on this camera model.
First, LENR only works on exposures of one second or longer. The way it functions may cause some of the symptoms you've described. With LENR on, your first 1-second or longer exposure is taken normally, then LENR will automatically take a second, "blank" or "black" exposure of the same duration with the shutter closed. So, if you are taking a 15 second shot with LENR on, the camera will immediately make a second15 "shutter closed" exposure, taking a total of approx. 30 seconds for the two exposures.
The second exposure is used to identify image noise, which is then "subtracted" from the first image. While it is being made, the camera will flash "busy" and most of the controls won't work. It can only be cancelled by turing the camera off at the switch or by removing the batteries, as you've been doing. However, if you cancel LENR mid-exposure, both the first and second exposure are discarded... no image is saved to the memory card.
On 7D LENR has three settings: Off, Auto and On. You should try with it "Off", if your exposures are running 1 second or longer.
You say you've already turned off all noise reduction (BTW, in my opinion it's not a good idea to turn off the other type of NR... I'd leave High ISO NR set to Standard, for example). It's a pretty remote possibility, but maybe the LENR setting is stuck "On" or in "Auto". Maybe for some reason it wasn't cancelled when you turned it off. You might try resetting the Custom Functions. (Warning: this also will reset all other C.Fn that you've set, back to the factory default setting).
Somtimes it helps to "reboot" the camera itself. This is much like rebooting a computer to clear the memory. With 7D it's fairly simple to reboot. Just turn the camera off, remove all the batteries (including the small silver date/time in the drawer inside the battery compartment), turn the camera on and press the shutter release button once. The shutter won't actually fire, of course. What this does is drain off any and all power that might be remaining in the circuitry after the batteries have been removed. It forces clearing the memory, same as rebooting a computer. Turn the camera off again, put all the batteries back in, turn camera back on and check the date/time. They should need to be reset, if you got a proper reboot.
While you have it out also might be a good time to replace the silver date/time battery (it's a CR1616 or something like that). Those have a typical service life of about 5 years and are relatively easy to find and cheap to replace.
Another thing you might try is reinstalling or updating the camera's firmware. It can be downloaded from the Canon website and is fairly easy to do, though you have to be careful to do it properly. Just follow the instructions carefully.
There's no guarantee that rebooting or fresh firmware will solve a problem, but sometimes it does and they are relatively easy things to try yourself. If they don't work, well you aren't out much other than a little of your time.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER
