Mac os on external drive

I have 1tb external drive on which i want to install mac os. But i only whant to use 250gb of it because i have some files on it that i need but cant move them somewhere else. How to install mac os only on 250gb of disk without losing data from disk? And also if i install mac os on external drive will it just appear in bios everytime i plug it in or it will be listed in bios forever?

submitted by /u/Im_Mefju
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