Side-by-side specifications of the Synology RS2418+ and RS2421RP+, from official datasheets. Differences are highlighted.
| RS2418+ | RS2421RP+ | |
| CPU | Intel Atom C3538 | AMD Ryzen V1500B |
| Cores / threads | 4-core | 4-core/4-thread |
| RAM (base) | 4 GB | 4 GB |
| RAM (max) | 64 GB | 32 GB |
| Drive bays | 12 | 12 |
| Expandable to | 24 | 24 |
| M.2 slots | 2 | optional via adapter |
| Network | 4 x 1GbE | 4 x 1GbE RJ-45 |
| 10GbE | optional | optional |
| Max raw capacity | 240 TB | 108 TB per volume |
| Hot-swap | yes | yes |
| Form factor | 2U rackmount | Rackmount 2U |
| Released | 2018 | 2021 |
Pick the RS2418+ if you want the cheaper, lighter box: it has the Intel Atom C3538, 4GB RAM with a 64GB max, 8 bays, no built-in 10GbE, and no M.2 slots. Choose the RS2421RP+ if you want more headroom and redundancy: it uses the AMD Ryzen V1500B, supports up to 32GB RAM, still has 8 bays but adds a redundant PSU (“RP”), and also has no built-in 10GbE or M.2.