Hi bandela,
Welcome to Sony Community!
All electronic devices cause heat when over used but if you are asking if connecting an external hard drive will cause any stress on the laptop then the answer is no because your files will be going to and from the external drive and not from the laptop.
For further assistance regarding your concern, please contact the Sony offices/Sony representative offices nearest to your place of residence in Asia Pacific region http://www.sony-asia.com/countryselector.html?hpid=countryselector:AsiaPacific. Due to proximity, they are in a better position to respond to your questions or concerns.
Regards,
Jen
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