At Western Digital the most clear color classification of hard disks therefore it also becomes a story basis about what HDD are presented today in the market and as them to pick up for the needs. In total color series four: black (black), green (green), blue (dark blue), red (red). Thus outwardly these disks differ nothing. 
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At office or houses best of all are suitable for daily needs "dark blue" disks. They have that optimum combination of the price and functionality which suits the majority of users. Actually, it is more about them and there is nothing to tell. If there is no need to work with graphics and video, to play games, and also there is no desire to pay much, a hard disk with blue mark from Western Digital – most that. Thus not the cheapest decisions as are rather high-speed.
At other producers I did not find analogs of a "household" series (that is other companies do not mark specially such disks), but at Seagate it is traditional Barracuda from the bottom and average price range, and at Toshiba – anonymous while the disks created on the basis of Deskstar (Hitachi). Samsung does the Desktop series, but it (according to Koreans) is suitable literally for everything, including downloading files through clients of BitTorrent. Such positioning looks a little strange as standard stores of data for «and office» are not recommended to be used the house in a mode 24/7.
"Green" WD is hard disks for those who thinks and cares of environment. Disks differ the reduced power consumption, but a payment for it – smaller speed of work. Frankly speaking, HDD with green mark (they are and at Seagate, and at Samsung a similar series is called as Eco-green) serve economy of energy as useful to the nature all of them equally do not do anything more. And are made on the same technologies. Unless do not contain some lead according to requirements of the instruction RoHS existing in the European Union since 2003. And still "green" stores are good for putting in laptops where each percent of spent energy of the accumulator is important.
The "black" hard disks Western Digital are a solution premium. High-speed, reliable, expensive. They find the place in computers of gamers, corporate servers or workstations for video installation.
«A red series» from WD is the newest and most disputable line of stores. According to the producer it is intended for NAS – network stores which swing torrents day and night (not necessarily, of course, torrents, but generally for this purpose use them). And at many users the role of NAS carries out the ordinary computer working constantly. Here for a similar operating mode of WD Red also are created. However, many forum experts christened at once "red" marketing at its finest. The pier, and a usual disk is capable to work for years day and night and anything with it does not happen. And on the contrary, "red" HDD has every chance to depart in work half a year-year.
Marketing or not, but is one vital circumstance, knowing which, it is better to think well what to load into the network store for a torrent downloadings. If the usual disk "departed" in a mode 24/7, the service center can refuse its warranty service or replacement. After all producers accurately recommend «office mode» for usual hard disks: work within 8-10 hours in day with a long break for the night. And here the "red" disk is specially created for unceasing downloading and a perekachivaniye. Thus to choose WD Red for the ordinary computer, too not the best idea. It is more expensive standard storage of data, and its efficiency and reliability in any way will not prove at standard loadings.
Similar development for BitTorrent is and at other companies. Adherents of Seagate will accept models with NAS abbreviation in the name, and at "Toshiby" it is necessary to look narrowly at decisions for servers. In fact, WD RED also is a server solution with heating control, protection against failure, etc.
And tests of disks of various producers, and also comparisons of different lines of one company on the Internet there is a lot of comparisons, and it is possible to find. Knowing the main HDD Western Digital types, it is possible to decide quicker on the necessary disk, to compare it to analogs at Seagate, Samsung, Toshiba, Asus or HP (producers of traditional HDD remains ever less) and competently to spend money for purchase.