Why do some HDD caddy's need power supply & others dont?

Solution
2.5" drive caddy:
USB3 don't need it since USB3 can accommodate 900mA
USB2 need it since USB2 can only accommodate 500mA which is not quite enough for the 2.5" HDD, so you either use a power supply or a Y-cable
eSATA need it since eSATA port is data only, no power

3.5" drive caddy:
all need power

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2.5" drive caddy:
USB3 don't need it since USB3 can accommodate 900mA
USB2 need it since USB2 can only accommodate 500mA which is not quite enough for the 2.5" HDD, so you either use a power supply or a Y-cable
eSATA need it since eSATA port is data only, no power

3.5" drive caddy:
all need power
 
Solution