Can I use an external hard drive as my main hard drive for the MacBook Pro?

Please note these replies are not from Apple - but users. So be patient please.

What is that 400gb on the MacMini? Can you just load that onto an external drive - then migrate your old computer to the new MBP?

You're comparing two dramatically different things between a 1TB HARD DRIVE (HD) and a 1TB SOLID STATE DRIVE (SSD). Hence the large price difference. You won't be able to install a drive you buy from best buy internally - I think that model is soldered to the motherboard and not user replaceable. Doing so would probably void your warranty.

You do NOT want your primary boot drive for the computer to be an external drive, this will be slower and immensely more complicated than needed. Sounds like you'll need to keep your large files on external drive(s). Likewise only having 1 hard drive copy of anything is not good data management/backup/workflow. So you'll want your files backed up to at least 2 drives (if not cloud/offsite storage as well)

Good luck!

Be careful when buying external drives. Many of the cheaper ones have the USB cable hard wired to the drive itself. This means if anything happens to the cable, then you will not be able to easily access the data on the drive. Make sure to purchase an external drive with a removable/replaceable USB cable. Make sure you backup your data to at least two different devices.

First, the idea of lugging around an external hard drive with a notebook defeats the entire purpose of having a notebook. Of course it is possible to curate your data so that you can fit your really important items on the notebook's internal drive and keep the rest on an external that you'd mostly keep on your desk. But if you intend to use the notebook the way notebook's are meant to be used and boot from an external drive just stop.

Second, there's a massive difference between a $50 1TB rotational hard drive and an SSD which is incredibly faster, uses less electricity and not affected by bumps and jolts. Yes, an SSD is more expensive than a rotational drive and yes Apple charges a premium for its SSD storage. However, if you look around the retail market you'll see a wide variation in SSD prices and if you look closer the price differences revolve around speed and reliability. I'm not saying a Crucial Tech MX500 series SSD will never fail but its 5 year warranty speaks to their confidence in it.

Third, Migration/Setup Assistant does give you some flexibility. You can select just your Applications folder to migrate, you don't have to migrate your account. Or you can migrate your account but deselect some of the folders (top level only). So you can omit the Music folder for example but you cannot keep some of the music folder but omit others.