This Zyxel is a NAS, a Network Attached Storage, a shared hard drive to use with all the computers in your network. It has two disk drives in RAID 1: if one breaks, there’s another copy, for safe and quick data recovery.
I bought one 4-5 years ago, and it’s a great device, but being on 24/7, the fan has degraded, first it become noisy, then it stopped working altogether! Without the fan, the CPU started to reach 75° C!
So, this is the fan they used in this device: it’s an ADDA AD-0612MX-G76, 12V, size 60x60x10 mm. This specific model costs 20 euro, too expensive. Anyway, since the connector is a standard 3 pin one, you can buy any similar fan.
I got this Startech fan for 6 euro, it’s more silent but at the same time it has a very faint but high pitched noise. Maybe another fan would have been better.
Moreover, because the CPU does not have any heatsink (!!! WTF) but it just has a fan nearby, i got also those small heatsinks, designed for the Raspberry Pi. The big one one the CPU, and four small ones on the RAM chips. Since I don’t really trust the thermal glue they came with, which could loosen with heat and touch other components, creating a short, I placed the NAS on a side, instead of keeping it straight.
Ps: I also have a Zyxel NSA325, but the fan size is different.