It's easy to fill up a 120gb hard drive. but the SD card slot is for example, digital camera memory card direct upload. For example, I use a mini SD card in my phone for pix and flix and music and I pop the mini SD card in it's slightly larger counter part bringing it up to SD card size and put it in the slot on my pc or laptop. Then I can take the pix fresh and load up on the internet or whereever in a snap, or copy music from my pc hard drive to my mini card, which windows explorer sees as a removeable media in Vista, as if it was just another drive. Then I can change the music on my phone or in the case of halloween I had scary sound effects, press the phone play button in my pocket and scary sounds came out of me. hee hee - I was a witch and occasionally screams or sounds would emerge from my pocket and whomever was standing near me at the party wondered "where is that sound coming from?" added to the mystery of halloween fun.
There are so many ways to transfer files music and pics now from pc to phone or digital camera to pc, etc. It can get confusing for some.