Check the answers to Where can I find computer hardware/networking equipment.

I think Sujata Computers and Data Care Corporation are good options.

Try Venus Traders, on FC Road.

I'll give the current price expectations since best prices change across the stores at times.
When you buy, pick a standard store, confirm a packed piece, get billing, and conform warranty.

You can get the Seagate 500GB "Go" edition in most places -- but check that you do not pay more than 4000/- for it. You might get it around 3500/- too.

The best price for a 1 TB Seagate GoFlex is 6300/-
I got that at Staples Mumbai!
Here you might see prices around 7000/- avoid paying more than that.
(fyi, the Seagate GoFlex can be scaled to USB 3.0 with an adapter).

For a 16 GB flash the price should be under 2000/-
Transcend is usually a good pick.

I've bought USB hard disks and pen drives from The Electronic Store in Aundh (Westend, near McDonald's). Obviously the price is not negotiable and probably not the lowest, at such places. But I prefer to support organized retailers who actually pay their taxes :-)

Try Sun Shoppe in Deccan. It is a small shop but the price is negotiable.
This is their website : www.sunshoppe.co.in/

I buy all my hardware at the places mentioned by Navin. In case, you do not know the locations: Sujata Computers : Nal Stop, Karve Road Data care corporation : Near Baba Bhide Bridge, behind Sukanta Hotel, off J.M. Road. It also has an outlet on Sinhgad Road, but the stock was not available for most items when I visited it.

Chroma , Aundh, thts where i buy most stuff, you get lots of choices there.

^ we had only bad experiences with chroma. they always have some old - show piece which they try to sell at market price. last time when we went to buy a mac for my friend, they tried to sell off an exhibited piece with 2nd hand adapters, without a box and wrong model numbers.