Presently there are 3 hard disk drive makers. The others, including Hitachi, have been incorparated into WD, Seagate, and Toshiba.
Here is an image of their mergers.
For availability, I'd purchase one from one of the main HDD manufacturers. If this is for a single network storage disk, or for a NAS where you may decide on a redundant solution requiring additional (the same) drives, buy from a company that will be able to supply the same drives in the future.