ill a byte array from DllStructGetData($lpBuffer, 1)

[jchd]

If DllStructGetData($lpBuffer, 1, $i) is too much to type and/or slow on heavy access, then copying the content of the buffer (using this very same function) to an array is a possibility.

[weirddave]

Each byte could get accessed 8 times as some of the data bytes contain 8 discrete digital signals, and the data isn't processed in order.

It's not immediately obvious which method is better, but I suspect:

FileWrite($hFileOpen, DllStructGetData($lpBuffer, 1))
For $i = 1 to 256
    MyArray[$i-1] = DllStructGetData($lpBuffer, 1, $i)
Next 

is going to be better than:

$data = DllStructGetData($lpBuffer, 1)
FileWrite($hFileOpen, $data)
For $i = 1 to 256
    MyArray[$i-1] = BinaryMid($data, $i, 1)
Next

(I've reduced the code down to the bare minimum for clarity)