It has a mind of its own!
 →  저마음대로야!

[예문]  
※  it (or one) has a mind of its (or his / her) own: 시키는대로 하지 않는다 제멋대로 굴다

A: This computer is impossible!
    이 컴퓨터는 구제불능이야!

B: What's wrong this time?
   이번엔 또 뭐가 잘못됐어?

A: It has a mind of its own!
   저 마음대로야!

B: May I help you?
  도와줄까?