I have connected a 2,5" HDD to my Raspberry Pi directly and it works.
The only one question I have - Can my HDD/RPi be damaged? (When I use the HDD without powered usb hub - Are there any risks?)
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It probably won't damage the RPi, but it may cause random crashes. Depends on many factors. Power supply, other periperals, cpu load etc.
The main polyfuse is evil. If the current is on the high side it could trip after a few hours even if nothing else changes
I don't power mine through the polyfuse anymore. Much more reliable.
The worst part is plugging in the hard drive. The initial jolt of power needed to spin the internal discs can cause the Pi to reset.
But if your is not then that is fine. When it is spinning the power consumption is stable and low, even when accessing the drive it does not go up that much.
If you have a Rev2 board and a good 2Amp power supply you should be OK. But be careful of adding any thing else. For example adding Wifi or 3G modem might just go overboard and start to cause issues.
If it's 7200 RPM, you might have a problem. Check the volts/amps on the hard drive label and compare it to the maximum USB outage. if it's marginally over it, that's ok, your hard drive should still function. Either way, if you happen to trip a polyfuse, simply wait a couple hours for it to cool down, then try to find a USB Hub.
Another route is to make like some USB DVD drives, which have 2 USB ports: one for power, one for data and power. If you're comfortable you can cut a spare cable and DIY it.