Searching and indexing of the catalog will see the best performance from being on the fastest drive possible: an SSD. If space is a concern, the proxy files that Photo Mechanic produces for the catalog can be stored in a different, potentially slower location. SSDs won't suffer the mechanical failures that an HDD might, but can certainly fail in other ways. The best way to ensure you don't lose your data is to perform regular backups, but the only true data loss that would occur if the drive storing your catalog were to fail would be Collections that you create in the catalog, all changes to the metadata of individual images are written to where the source file is stored.
Hope that helps!
