Checking an MD5 of a device
Using string concatenation in a loop is a no go because each time you do a += a new string object will be created because strings are immutable.  

To use the right tool for the job I would suggest to use a StringBuilder instead. Using a StringBuilder doesn't have this restriction because the passed strings are just appended char by char to the internal buffer.  

Using braces {} although they might be optional will make your code less error prone.  

By extracting the building of the hex string representation to a separate method the responsibility is split nicely and could be reused if needed. Having this as a extension method could serve well. 

private string ToHex(this byte[] values)
{
    if (values == null) { throw new ArgumentNullException("values"); }

    StringBuilder sb = new StringBuilder(values.Length * 2);
    foreach (var b in values)
    {
        sb.Append(b.ToString("X2"));
    }

    return sb.ToString();
}  


resulting in the former code like this  

var md5String = string.Empty;
using (var md5 = MD5.Create())
{
    var md5CRC = md5.ComputeHash(File.ReadAllBytes(destinationFilename));

    md5String = md5CRC.ToHex();
}