Side-by-side specifications of the Synology RS409+ and RS409RP+, from official datasheets. Differences are highlighted.
| RS409+ | RS409RP+ | |
| CPU | — | unknown |
| Cores / threads | — | unknown |
| RAM (base) | — | unknown |
| RAM (max) | — | unknown |
| Drive bays | 4 | 4 |
| Expandable to | 4 via RX4 | 4 (via RX4) |
| M.2 slots | 0 | 0 |
| Network | 2 x Gigabit | 2 x Gigabit |
| 10GbE | no | no |
| Max raw capacity | 8 HDDs | 8 drives |
| Hot-swap | yes | yes |
| Form factor | 1U rackmount | 1U rackmount |
| Released | 2009 | 2009 |
| Power consumption | 28 W / 9.5 W | 61 W / 29 W |
| DSM at launch | 2.1 | 2.1 |
Choose the RS409RP+ if you need redundant power supplies; otherwise the RS409+ is the same class of box for less. Both are older 4-bay units with no meaningful advantage in CPU, RAM ceiling, 10GbE, or M.2 expansion, so the decision is basically redundant PSU vs. lower cost.