portable hardisk desktop=3.5" (bigger, need extra power adapter to power it)
portable hardisk laptop=2.5"(smaller, only 1 usb cable is enough to power it, unless ur computer is old tat requires 2)
wd passport=same like portable hardisk laptop, but it has more features such as data encryption, n more colour for u to choose
any external hdd is portable as long as u have usb ports...for 2.5", some old pc needs 2 usb port to provide enough power for the hdd, while most of the new pc need only 1 port

seagate, maxtor, WD, buffalo...all these brands r quite good....if u r getting the 500gb or 1tb, pls consider do u need firewire? the external hdd that has firewire costs more than external hdd that doesn't have a firewire port....what's the difference? firewire allows faster transfer speed, which matters alot when u are transferring few hundreds GBs, as USB transfer speed is ALOT slower than firewire

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Added on March 26, 2009, 3:00 pm

Famous brands such as WD, Seagate, Maxtor, Buffalo have quite good qualities...but FYI, none of the HDD promises "no data deletion/corruption/last long", as these kinds of problems are random n sometimes unexplainable...if any of the companies say that "our HDD has no data deletion/corruption/last long", they would be sued n become bankrup...because in mass production, surely among hundreds of HDD, few units are not working properly...also, it depends on how u take care of ur HDD to minimize data deletion/corruption/last long