I'm reading bytes from the file, into a byte array
Dim input As New FileStream("scm_app.bin", FileMode.Open)
Dim bytes() As Byte
bytes = reader.ReadBytes(CInt(input.Length))
But how to indicate that I want to start reading into an array from the fifth index?
bytes(5) = reader.ReadBytes(CInt(input.Length))
That code is rather screwy. If you just want to read Bytes from a file then you don't need a BinaryReader. The FileStream can read Bytes for you. Normally you wouldn't even need that as you can read a whole file into an array like this:
Dim data = IO.File.ReadAllBytes(filePath)
If you want to read into a specific position in an existing array though, you can use the FileStream like this:
Dim data As Byte() 'The array to write the data to.
Dim startPosition As Integer 'The position in the array at which to start writing the data.
Dim filePath As String
Using fs = IO.File.OpenRead(filePath)
fs.Read(data, startPosition, CInt(fs.Length))
End Using
data is the array to write the data from the fil into. If your array is bytes then use bytes instead of data. In my answer, startPosition is the position to start writing the data. If you want to start writing the data at index 5 then you set startPosition to 5. Just as you did in your original code, I specified that the entire file should be read but, if you only want to read a specific number of bytes then specify that number instead of the Length of the FileStream.