C - Malloc and memcpy (memory management)

I'm a bit new to C and I'm having trouble understanding how memory works, especially in-built functions like memcpy.

Here's a struct I'm using

 struct data_t {
    int datasize;   
    void *data; 
 };

And here's an auxiliary function that I'm using it with:

struct data_t *data_create(int size)
{
   struct data_t *dt=malloc(sizeof(struct data_t)+size);
   dt->datasize=size;   
   dt->data="1234567890a";
   return dt;
}

Now in the main function I have no problem doing this:

struct data_t *data = data_create(1024);
data->data="123456a";//just an example

But this throws a Seg Fault:

memcpy(data->data,"123456a",strlen("1234567890a")+1);

My question is why? And how do I avoid it? Please bear in mind I'm new to C so how C deals with memory is a bit new to me

Thank you.

Edit: It works! Thank you very much. Completly missed the data pointer. Now everything is working fine according to valgrind.