Seek Time

I'm surprised that there wasn't any mention of the mechanical hard disk's Achilles heel: seek time. It takes at least 5mS (usually more like twice that) for a hard disk to change the cylinder address on the disk from or to which it will next read. In contrast, "seek" for a SSD is a matter of microseconds, if it means anything at all in that context.

It's my opinion that in the shorter term (i.e. the next decade) the most fruitful developments will be filestores and database stores that combine a SSD and a hard disk, so that the large chunks of data that are accessed sequentially reside on the (cheaper) hard disk, and the smaller, heavily and randomly accessed chunks reside on the SSD.

It can be done at the controller level, to avoid any need to introduce new system software. Put a few hundred Mb of fast flash memory on the circuit board of a hard disk storing a few hundred Gb, and let the disk controller use it as a cache. Unlike RAM cache, there is no problem if the disk drive loses power.

In short, it's not simple competition. These two technologies should be synergetic, at least until solid-state costs little more per Mbyte than hard disk.