First of all, it's really big. If you're looking for a case to fit many portable external HDDs, you may fit easily up to 4 (depending in the size of them).
In my case I bought it for a not portable HDD (the one you need to plug to the electricity to work). It really fits good, holds it tight, and thanks for the EVA materials outside, and the comfy-plushy interior, you now your HDD will be protected for storing or safe transportation (not a tough transportation, but much better than having to carrying the HDD without any case). It has space to also store the cables necessary for the HDD, and even extra space for one more little portable HDD.
Surprise for my sister. Protection and peace of mind!
I already own one. I KNOW of its quality and reliability. When my sister purchased a drive like mine I urged her to get on as well. She did not. So I did it for her. She dosen't know yet....... ;-)