The reason is simple. The camera or camcorder would have to support the drive or the drive would have to support the camera. This usually means the camera maker would offer such as an accessory.
I do not want to carry a laptop when I travel because the computer is heavy. But, I want to back up my video and picture into a portable hard drive while I am travelling. Is there any portable hard drive can transfer video or picture from video camera and digital camera without connecting to a pc?