I have always done this with 3.5" external hd. I connected 10 harddisks (3.5") to raspberry pi. Will I have problems if I do it with 2.5" external disks? 3.5" disks are powered from external power. This is ultimately powered only by the raspberry pi. Will I have a problem using 2.5" disks on Raspberry Pi?
If you are going to connect more than a single drive, you really should provide them with external power and not drawn all through the USB port.
i am connecting the 2.5" disks to a usb hub with power supply. Do you think this is enough? I will connect 3 usb hubs to raspberry pi. (usb hub device has power input).
A powered hub should not draw power from the Pi. Read the tech specs though on how much it can handle, and look at peak draw for all devices spinning up at once.