F4-424 M.2 drives for OS & cache

[agarza647]

After much research on nascompares.com, I have finally settled on the F4-424. I plan to install two 12Gb drives initially and some M.2 drives for OS storage and cache. The NAS will be used for backups (Mac & PC), multimedia (Plex), file storage (both general and video editing), and possibly VM/Docker usage in the future.

1. What would be the recommended drives (brand and size) for the two slots?
   
2. I realize that the M.2 for OS storage would need to be installed initially, but does the M.2 for caching need to be installed at the time of initial setup also?
   

Thank you for supplying the community with such a wealth of information and resources.