EEG data in same array

Hi, I am using matlab to do the online processing for EEG signal.My problem is how can I save the data since the data of EEG using the same array(ex,matrix_data). The data is transferred by block which means 1s=512(no of samples). This means the data is renew in every iteration in the same array. so, if I want to save the data for 10s,how should i do it? Really need help?

Thank you.

[Walter Roberson]

How are you reading the data?