Toshiba did not buy Hitachi's hard drive business AFAIK. WD bought Hitachi and had to sell
some of its 3.5" manufacturing assets to Toshiba as demanded by regulatory agencies in order
to avoid the 3.5" HDD market becoming a duopoly between WD and Seagate.
WD also bought some of Toshiba's Thailand facilities in the same deal.
Source: Press release 1 by WD.
Press release 2 by WD
Regarding the Hitachi drives: What is today known as the company making Hitachi drives (HGST)
was founded by the merger of IBM's and Hitachi's HDD businesses in 2003. AFAIK they have since
been more focused on the enterprise market than the consumer market, and consequently make
some very good drives from what I've heard. But they do cost a bit more than others.
As for failure rates: Often when multiple drives fail those drives come from the same manufacturing
batch. If you buy several drives at once they are likely be from the same batch, and therefore
if one drive fails for manufacturing reasons the chances that you'll have several failures is
higher. I've read horror stories about people with mass drive failures from all major manufacturers,
I don't really see a dependable and verifiable reason to distrust one more than the others at
this point. If there really were serious problems with one manufacturer's drives on large scales
we would be hearing a lot more than just the occasional anecdote, considering how many millions
of drives they all put out. At least IMO.
