I've had pre-production WD PI DRIVE for testing and development for a couple of months. So here's a few numbers.
speed:
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PI DRIVE = 30MB/s
SD CARD = 20MB/s or slower (depends of the card)
ETHERNET = 10MB/s
pros:
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1. GOOD DEAL
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but only if you want to buy usb hdd anyway, because the HDD itself is not that different than regular USB hdd that you may have in a drawer
2. SPECIAL POWER CABLE
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This one is brilliant, one cable that powers raspberrypi AND the drive itself, this way you are not powering HDD from rpi usb port that is not reliable even if you enable high current in settings.
I've done A B test with that cable and without - and with THE SAME POWER ADAPTER (2A): with the cable - everything is rock stable, and when powering from usb it's not (the diode on rpi flashes indicating that there is not enough current for rpi).
3. HDD lower power consumption
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this is not something I can test - WD claims that this HDD is specially tuned to be less power hungry.
CONS:
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NOT FASTER THAN ANY OTHER USB DRIVE
this is still attached to weak USB 2.0 on rpi - so you don't have any speed bump when compared to off-the-shelve usb HDD
THE CASE IS FLIMSY
the base is ok - I'm using one to hang my rpi with HDD on the wall
but the outer shell is too thin for my taste and lacks top cover - so It's not really a solution for media player