External hard drive for iPhone?

Hi,

Very new iPhone user here, I am curious to know if it is possible to connect an external HD like a memory stick or a Western Digital My Passport to the iPhone via a USB connection.

I had a bit of a search around and came up empty handed which suggest it's not possible. Anyone able confirm whether it can or can't be done?

Thanks
Splint

I don't like to say "impossible," but let's just say it's outside of practical reality.