The only difference between internal and external hard drives is an enclosure. You can purchase external USB enclosures for internal drives and make them external USB drives. Enclosures for 3.5 inch drives should include a power supply. Enclosures for 2.5 inch drives often don't, but most of them don't need a separate PSU and run just fine from USB power (the exception would be 7200 RPM drives, which are more power hungry).
For 2.5 inch drives you can also get a USB-SATA interface cable that looks like this.
Which won't offer any drive protection, but may be cheaper (and an SSD might not need protection).
I boot one of my Pi3B computers directly from a 128GB SSD with that cable.
If you aren't planning on booting directly from the HDD, then just about any USB enclosure should work.
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?