System volume root's usage is 90% RN102

Be careful here, as there are support implications in trying to fix this yourself.  Typing the wrong thing can do damage, and require you to do a factory default to recover.  So I recommend backing up the NAS first.  https://kb.netgear.com/30068/ReadyNAS-OS-6-SSH-access-support-and-configuration-guides

You first disable the AntiVirus service - which it sounds like you have already done.

The second step is to enable SSH in the NAS web ui in system->settings.  If your password is still set to password, you also need to change it to something else.

Now you need a program to connect to the NAS using SSH.  Windows 10 has a program built in (called ssh).  So do Macs (terminal).  For other windows versions you need to install putty from https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html  (and you can also use putty on Windows 10 if you like).

Then you use that program to connect to the NAS:

1. ssh root@nas-ip-address on Windows 10 (entered on the Windows 10 search bar)
2. launch terminal, enter ssh root@nas-ip-address on the mac
3. launch putty, make sure ssh is selected as the connection type. Enter the nas-ip-address and click open.  Enter root as the username.

Use the real NAS IP address of course. It's important to log in as root.  Don't log in as admin.   When you see the password prompt, enter the NAS admin password (root and admin use the same password).

If you have trouble launching one of these programs, then you can post back (or just google for more info).

Be careful on the typing of the commands below. Be sure to use the correct slash direction, etc.

After you are logged in, you enter

cd /var/lib/clamav
ls -lsh

You should now see a folder listing, with some folders that look like clamav-<very long string>.tmp

If you don't see these tmp folders, then post back and tell us what you do see.

If you do see these tmp folders, then you remove them by entering

rm -r clamav-*.tmp
ls -lsh

You should now see a second folder listing, and those tmp folders should be gone.

asimb has posted a screen shot illustrating these commands:  https://community.netgear.com/t5/Using-your-ReadyNAS-in-Business/System-volume-root-s-usage-is-xx-This-condition-should-not-occur/m-p/1722860#M178464

I'm taking the liberty of re-posting that screen shot here.

I recommend leaving the AntiVirus service off - at least for now.