How to remove bytes from a specific index from a byte array

If speed is critical, you can create two new arrays and use Array.Copy() to copy the required bytes into them.

To make this easier to use, it might be more convenient to write a little extension method for arrays which extracts and returns a subset, like this (note: error handling omitted for brevity):

public static class ArrayExt
{
    public static T[] Subset<T>(this T[] array, int start, int count)
    {
        T[] result = new T[count];
        Array.Copy(array, start, result, 0, count);
        return result;
    }
}

Then you can use it like so (I have corrected the index from your example; you had it starting at 201, but it should have been 200):

var array1 = new byte[1000];

// ... populate array1 somehow, then extract subsets like so:

var array2 = array1.Subset(  0,  50);
var array3 = array1.Subset(200, 800);

// Now array2 and array3 are two byte arrays 
// containing the required bytes.

This is probably the fastest you are likely to get.

You could use IEnumerable Take and Skip, for example

byte[] origin = new byte[200];
for(int i = 0; i < origin.Length; i++)
    origin[i] = (byte)i;

byte[] first = origin.Take(50).ToArray();
byte[] second = origin.Skip(100).Take(50).ToArray();

Below code can be used to split the byte array, "index" starting index and "count" is the index count upto which index you want to split, here for your condition index=50 and count is 150;

List<byte> byteArray = array.ToList(); //array is the byte array
byteArray.RemoveRange(index, count);
Byte[] array1 = byteArray.ToArray(); //array1 is the resultant byte array

Cant you get the sub arrays like this?

byte[] array1 = array.ToList().GetRange(0, x1-1).ToArray();
byte[] array2 = array.ToList().GetRange(x2, array.Length- x2 - 1).ToArray();

where x1 and x2 is 50 and 200 in your example