Thanks for the pic; your unit is an RNS510.
Unfortunately, many of these units are starting to fail due to age; the old ones have HDDs that wear out.

Check out these threads.
Updating the firmware - I would try this first.

Firmware updates for RNS510 units in Australia/NZ

If this does not work, I would start looking at options to either upgrade the unit or replace the HDD.

RNS-510 HDD replacement/SSD swap DIY