Synology produces a number of NAS products. These products provide 1 or more bays for hard drives and generally 2 USB ports for adding an external hard drive or using a USB hard drive.
Although the Synology NAS can be expanded with USB flash media it still must have a physical hard drive installed to operate. Without a hard drive the NAS prompts you with a screen to install a hard drive and will not let you continue without one. In short, you can’t use a Synology NAS with only flash media.
It should also be noted that the flash media cannot operate as a RAID or mirror option and only as free standing space.