Question about Synology DS423+

It is not on the list of officially supported drives ( https://www.synology.com/de-de... ), but should fit as the interface is standardised. But since both 2.5" and 3.5" hard drives fit in the space, it makes little sense to take the smaller ones (unless the small drive is already present).
And don't forget that there are also two M.2 2280 NVMe SSD bays that only accept SSDs. This could leave the 3.5" bays free for larger (physically and capacity) HDDs, which are cheaper and more durable than SSDs. The Western Digital and Seagate models would be recommended.

Hello Haebistrau,
On most newer Synology models, the use of "unsupported" HDDs or SSDs will result in a warning in the storage management. As ÁdámZ has already mentioned, the list of supported HDDs, SSDs, RAM modules and expansion cards (Hardware Compatibility List, HCL) can be checked directly on the manufacturer's website.
I myself also use HDDs and SSDs that are not listed in the HCL. The corresponding warning message can be deactivated using a "hack" until the next DSM update. However, I would advise against doing this in a business environment, as Synology could refuse support in such cases.
If there are any further questions, I recommend consulting a local dealer or expert.
Best regards
Aedu