Question about Synology DS124

Yes, you can install an SSD hard drive such as the WD Red SA 500 SATA SSD 4 TB in the device. The NAS supports both 3.5 inch SATA HDDs and 2.5 inch SATA SSDs.

Theoretically, you would need a separately available holder for 2.5" SSDs. But as these are very light and do not vibrate, you can probably just plug them in, possibly secure them with an adhesive strip, that's all you need.