It is just the same as OpenFile behind the scenes so nothing to do with CFile. Your call probably fails because you are not including the share flags in your call to CFile::Open(), which are required when trying to access a device.
As others have pointed out, the issue is with pBuffer not associated with any allocated memory.
In addition to this, I would like to point out one more flaw in your code, although it doesn't matter in this case.
The third parameter to SetFilePointer[^] must be an address of a LONG variable. PLONG does not declare a LONG variable. It's only a pointer to a LONG variable.
Here SetFilePointer will actually try to write to memory 0.
What you need to do is declare a LONG variable and provide its address -
LONG highValue; SetFilePointer(..., ..., &highValue, ...);
You can also pass a nullptr, if you're not intersted in that value.
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Microsoft MVP (Visual C++) (October 2009 - September 2013)
If you are referring to array cells with two dimension values then you should use a two-dimensional array. As it is you can pass any values in to NodeCoord but there is no way of telling if they are valid. So you could end up with lots of random memory corruption.