maybe that's coz it's just wayyy too underexposed.
underexposed or decent exposure is not the problem here. at iso6400, i can still see a very dim image on the times it work. on the ones that show Err. the image is black. black as in "lens cap on" black.
LOL...i can understand why you're saying that. though this is my first dslr for a week, I've been using dslrs for a long time.

my problem now is, sometimes the mirror/shutter does not complete the whole cycle. let's not talk about underexposed or not. what i'm getting is black. total black like as if u had the lens cap on.

this Err problem happens intermittently but quite frequently. on the times it snaps correctly, I get an underexposed image or a normal image. on the times it fails (Err), i get a "lens cap on" black image.

now it doesnt just happen at 1/8000, it randomly happens at lower shutter speeds like 1/60, 1/40. ISO is at auto, so on the times it work, i get a normal image, on times it fail, i get black image but exif still shows the actual shutter speed i set.

when top LCD shows Err. i switch it off, took out the lens, saw the mirror was up and the shutter was covering the sensor. put it back, switched it on, press the shutter button again, and i can hear the mirror flip down. now I can continue snapping.

sorry if this is real long but i cant really think of a better way to explain this as I dont have a video of it. 

Did you by any chance set the mirror lockup, mirror up for cleaning, or shutter delay function, either accidentally or otherwise? Or maybe assign a custom button function to these settings and accidentally pressed it?

If not... then it would appear to be a mechanical issue. You can try taking pics with the lens off body and have a visual look to see if and when it happens, what actually happens.

See Page 306 of the User Manual.

Indicator: Err (blinks)
Problem: Camera malfunction
Solution: Release Shutter. If problem persists or appear frequently, consult Nikon-autorized service representatives.

When you send to Nikon, show them this in the User manual to help explain your case.

my fren's D90 also have the ERR problem. tis happens when she was taking pictures n din notice the long shutter exposure and when she hit the shutter button, it stays open for a short period n she turn the cam power off. since then, it give ERR when she took pictures at 18mm. i try to take out the lens and put it back, so far i haven't hear anyhthing from her. could this "mirror lockup during exposure" cause the cam to hit ERR?

^^

Looks like its a real problem. I guess its SC for you... I'm so sorry about your camera.

nope. definitely not mirror lockup. see my other replies.

tried without lens, without memory card. still same.

---

Added on December 29, 2010, 3:51 pm

   
it's only a week old. took more than 1k shots since i got it. but it was indoor event so no fast shutter. ytd, tried 1/8000 and then started like that already. now lower, medium shutter speed also like that. 

what other reason for this? not familiar with d90   
lens? shutter? or straight to SC? 

Yes, this is 85mm on FX. Meaning something like 50mm on DX.

This Agnes was playing around -.- So hard to shoot her LOL. Cause she off duty then came to say hi to me while I was shooting the Mitsubishi side. Then when I wanted to shoot her she was jumping all over .

Ah! I feel you bro. Especially when she gets over your MFD range. No AF confirmation sound = FRUSTRATION!

The models will wonder... what the heck is the photographer's doing.

d3100 can record sound right? but just that it cannot have external mic?

d3100 can record sound right? but just that it cannot have external mic?

can even record down the focusing sound of the lens...haha

can be called macro sound recording?