DriveDx is a hard drive and SSD diagnostic and monitoring utility from BinaryFruit. Features include monitoring of SMART data, temperature and free space with thresholds and algorithms for predicting problems, logging of data over time, email notifications of status updates, the ability to trigger self-tests, save detailed drive health reports, and more. Among other tidbits, the app provides details on sector size, which can play into compatibility issues. (External FireWire/USB monitoring is provided via the third-party SAT SMART Driver for devices that support SMART data transfer.)
DriveDx 1.9.0 is priced at $19.99 for an individual license for Mac OS X 10.6 and up with a 10-day trial period. (A $39.99 Family license supports 5 computers in a household.) The latest release brings support for a raft of new SSDs, among other updates.