You can, but it is not recommended. The HDD is configured into our system in a way that if you replace it, you will change the configuration. If you are a Linux expert, you can fix it, but if you are not familiar with ssh and Linux disk management, you can make the box unusable.
There is no default administrator password. When you start using your server, the first step is to create your own administrator password. Please do not forget this password, otherwise you will loose access to your device. The administrator's user name is "root".
Yes, you can. Start your client application, connect to the server, then open a terminal. Server address is the VPN server's address, user name is your user name on the server. Please do not try this if you don't know what is ssh or Linux shell.
It depends on for what reason you need the server. If it is only for yourself or your family or friends, 500MB memory is more than enough. If you plan to have more than 4-5 active users on it, you probably need more memory.
When you reset your server, it goes back to its original factory configuration, which includes changing its network configuration to DHCP. Use our apps' network discovery function to find your device on your network again.