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Two main uses for external HDDs:

  • Library too large for internal HD/SSD. So you put your photos, movies, music...etc. on an external library drive. Only carry around in a Macbook what you need, not everything you own.
  • Backup, backup, and backup. Use Time Machine or other app to backup both the internal HD/SSD and and any external library drives.

So you at least one large drive for backups and perhaps a second external smaller drive for libraries that do not need to be on the internal HD/SSD.
 
would it be good idea to get a External HDD for college or pass on it?

Very important. The chances of your computer dying is inversely proportional to the quality and timeliness of your backup per. Murphy.

Keep it on your desk in your room and plug it in while you're there. Set time machine up to backup to it. Cheap insurance.
 
It's certainly a good idea, but probably not as important as offsite backups. Personally, I never bother doing full system backups. I just backup the files I will want to have if I have to reinstall the OS. An external HDD is certainly useful, but can be stolen, dropped, flooded, etc along with my computer.

My university gives each student 25 GB of Google Drive space, as well as 1 GB of space on an SSH server. I encrypt all my research related data locally and send it to both of those. Documents, PDFs, etc don't generally take up too much space.
 
i think I'm a return my hdd , well one $70 is a lot to a college student lol:D , and two i can just save my important stuff docs on the drive and if i need more i think its like $1/month for 100gbs

also i have a 2014 retina mb 15" with 512 gb ssd , and the chances of my ssd failing a relatively low to my screen ghosting or some other problem
 
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