External Hdds or portable Hdds are available either with the standard USB2.0 connectivity option (some with USB 2.0 and firewire 400) or with eSATA (wich is way faster than the prev. 2 i.e USB 2.0 and firewire 400 (a.k.a 1394a)
if u go for external hard disk upto 500GB, then it will NOT require a external power adapter and only the USB port will be required to power it up and also transfer data b/w the hdd and the PC/laptop
if u go for external hdds more than 500GB like say 750GB or 1TB (all 1TB drives come with a power adapter to power it up for data transfer) and also come with eSATA port
if speed (b/w transferring data) is not a priority, then u can opt for a 500GB external hdd with USB2.0 connectivity, wich will also reduce the hassle of carrying a power adapter along with it, but as its USB2.0 the data transfer will be damn slow if u are transferring large chunks of data in the range of 7-8GB per day, will take close to 2hrs (With eSATA, its like i hv done 100GB data transfer in 45mins flat) so u can see the difference...
if speed (b/w transferring data) is a priority and also the space, then u can opt for a 1TB ext hdd with eSATA (provided ur PC/lappy has an eSATA port, ofcourse)..
most of the brands are gr8, i recently purchased a Seagate BlackArmor WS 110 (1tb) ext hdd) With eSATA (the 1TB and eSATA were my main requirement) at the price of Rs.6.8K and its blazing fast and really working gr8... (u can see its pics in the core ix thread , link in my siggy)
so the choice will depend on u regarding:
1.) how much data u wanna transfer,
2.) if speed is a priority or no (wen transferring data more than 10+ GB at a time),
3.) ur budget.
as u hv mentioned tat u want to transfer (i read it at the last moment, sorry for tat) videos, music, data files etc.. then since its NOT tat high of a data transfer (as compared to game ISO's and/or hi def 1080p movies) u can go in for a USB2.0 powered 500GB ext hdd, that wud (i think) solve ur needs..

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Cheers n e-peace.....