As of today the biggest Portable External HDD is about 750 gb, vs the 1,2,4 and even 8 TB non-portable external HDDs.
The differences, besides capacity, are two:
1. Portables are powered via USB, non-portable are powered through an AC adapter (although non-portables does not mean you can't take them with you because they weigh barely above 1 pound and you can use them anywhere you have a power outlet).
2. Speed, USB 2.0 (which most portable HHDs are) raw data speed is about 480Mbps. Non-Portable SATA HDDs are 1.5, 3 or 6 Gigabit when connected to a SATA connection on a motherboard... if you use a SATA Non-Portable external HDD no matter how fast the HDD is you will only be able to transfer data up to 480 Mbps so all that excess of Hard Disk speed is wasted.
Although, if you use an Ethernet connection to transfer data to a Non-Portable external HDD then you will be able to get the most out of it's speed.
Bottom line, if you are thinking on getting one of those think about your necessities and choose between mobility or speed, to put this in perpective a 3 MB mp3 song will transfer in about 1/20 of a second to a portable HDD.
So choose wisely...