Converting long to byte array and back

I need to convert a long to a byte array and back again. I can't use BitConverter since the Micro Framework version of C# doesn't support it. Here's the code I'm using:

            long origLong = long.MinValue;
            byte[] bytes = new byte[8];
            for (int i = 0; i < 8; i++) {
                bytes[i] = (byte)(origLong >> (i * 8) & 0xFF);
            }
            long newLong = 0;
            for (int i = 0; i < 8; i++) {
                newLong = newLong | (long)(bytes[i] << (i * 8));
            }
            for (int i = 0; i < bytes.Length; i++) {
                Debug.Write(bytes[i] + " ");
            }
            Debug.Print("");
            Debug.Print("orig: " + origLong + ", new: " + newLong);

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The results in the Output are:

0 0 0 0 0 0 0 128 
orig: -9223372036854775808, new: -2147483648

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It looks like the conversion to a byte array is good but the conversion back to long is not. Also, on the statement with the OR operator (#8), I get the warning "Bitwise-or operator used on a sign-extended operand; consider casting to a smaller unsigned type first".

How can I do this?