Replace WD green drive with red/black drive

Per the Aconis knowledge base (https://kb.acronis.com/content/9136), the multi-zone errors are something to watch, but not a sign of imminent failure. If you would like to replace it, then here's my take on red/green/black.

The 1 TB red (WD10EFRX) is on the HCL. The 2 TB red (WD20EFRX) is not but is for most other ReadyNAS models, and it works fine. I'd definitely chose a red over a green (no matter what the HCL might say). I have greens in my NV+, but as they fail I'll either retire the NAS or replace them with reds.

Blacks are (in my experience) noisy, and they will run hotter than your greens. However, they should work fine (and the WD2002FAEX is on the HCL). I have no strong opinion on whether blacks or reds are better for 24x7 use - though I use Reds extensively and they've worked out well for me. There's also the WDC Red Pro (WD2001FFSX) - which isn't on the NV+ HCL, but is for the newer NAS (so I'm pretty confident that one will also work well). In any event, the normal reds are intended for NAS, are just as "green" as the green drives, and will perform at least as well.

Back to your main issue, if you are getting dropouts when copying large files, then perhaps your OS partition is starting to fill. If you have ssh installed, then you might want to check for that. If not, then perhaps delete all logs (backup and system) and see if the issue goes away. Don't download the logs first - the system tries to create a zip file, and if the OS partition is filling, that can lock up the NAS and prevent it from booting..