If you are looking for the largest portable hard drive, one that can be powered by a USB port only, then check out the WD Elements Portable 6TB model available direct from Western Digital.
At $169.99, this is cheaper than buying it from Amazon, and it comes with 2-year warranty as well as the option to add a 2-year data recovery plan for a mere $9.99, a must have in my opinion.
The 6TB model is much thicker than the lower capacity models (up to 57%) to accommodate for the taller internal hard drive and the extra platters. Like older versions, it comes with a now-obsolete flat Micro-B USB connector and USB 3.2 Gen 1 speeds (5Gbps).
It doesn’t come with any software though, which means that its slightly dearer sibling (My Passport 6TB, $179.99) may well be a better buy. That one has a three-year warranty, 256-bit encryption via the included Acronis True Image software and a cheaper 3-year data recovery plan that costs only $7.50 at the time of writing.