At 10.8.1, I have an 32-bit, unsigned, integer TIFF raster. The NoData value is, as expected, 4,294,967,295 (2^32-1).
The values in this raster range from only 0-7, which makes a 32-bit depth overkill. I thus decided to use the Copy Raster tool to reduce the bit depth, using Pixel Type = 4-bit. The tool help states that the output will be an "...unsigned integer. The values supported can be from 0 to 15."
However, the tool's default output NoData Value is -1. Sure enough, when using this default setting, the output is an unsigned 4-bit TIFF, where the NoData value = -1.
As far as I know, unsigned rasters can only contain positive values. Since my output TIFF is unsigned, I would thus expect its NoData value to also be a positive integer, just as in the source 32-bit unsigned integer TIFF.
How can an unsigned integer TIFF contain a negative NoData value?
I'm asking this question in order to better understand raster technology in general. I'm not asking how to "force" a postive NoData output value, which could be done by selecting a positive value in the Copy Raster tool, or by using the Raster Calculator, as commented by MoreMeowBell.