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Comments from our professional photographers welcome, but I don't think DSLR's concept of ISO has much to do with film's concept of ISO.
With film, higher ISO comes from larger grain in the emulsion.
With digital, it comes from how much amplification is applied to the readout from the CCD/CMOS. Double the "ISO" and the camera roughly doubles the readout value from each pixel.
You don't want to overfill the pixel wells and too long an exposure could do that.
With longer exposures you are probably subject to more amp glow.
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