The existence of one or more gods can't be conclusively proven or disproven. So it makes sense to me that some people believe in it and others don't.

I cannot recommend any USB-connected drive for long-term use. (Only for portable devices that get plugged in for a little while at a time.) In the long term, any USB drive will randomly reset during periods of heavy use -- including heavy writes, meaning some data will get lost.

USB enclosures tend to just crap out completely after a year or two, if used continuously on a server. I know because I twice used 1TB external drives with OpenWRT (home router) devices. The data will be safe on the drive, but you'll have to replace the enclosure.

1. My first recommendation would be to look very carefully at the chassis and see if there's any way at all to fit another SSD inside it. 2.5" SSD's are usually thinner than 2.5" hard drives, so it may be possible, and most motherboards have more SATA ports than they need.

Is there possibly an NVMe slot on the motherboard? Or an open PCIe slot where you could put an NVMe adapter?

2. My second recommendation would be using a 2.5" hard drive. Newegg has a 5TB one for $135, but unfortunately that's as large as they seem to go. It will be a bit slower than an SSD, but still probably around 150MB/s for sequential access.

3. My third recommendation, if money is really tight, would be an additional server, with a large 3.5" hard drive. This will be a lot cheaper than an 8TB SSD, but adds complexity, electricity use, space use, and possibly fan noise.

Actually, I think that's a really good idea. Just be aware of possible inductance in the long wires from the transformer area going to the camera. Putting a large electrolytic capacitor next to the camera should fix that.

As for your bell, there are dead simple systems that use a battery-powered stick-on button with a transmitter, and a chime/receiver that plugs into a wall. If you want to be fancy, you could find a way to power the transmitter with that 5VDC, in parallel with the camera... or you could just keep it simple and use the battery that comes with the button. They last near forever.

Remember that voltage is measured across a pair of wires, so you can't power the chime with only a single wire. That's part of what makes this difficult -- these doorbell systems only have the bare minimum of wiring in them. Powering the camera and the chime in series with each other is quite difficult. I think a lot of these things just accept the short circuit, and use a battery to power the camera while the button is being held.

Here is what I was proposing, and I think what ch00f was also proposing. Replace "5VDC power supply" in this diagram with "a full bridge rectifier and a bunch of caps" in their description, and also note that your camera probably requires a well regulated 5VDC supply.

The easy way: take your existing wiring. Put an 18VAC to 5VDC power supply in parallel with the pushbutton. Then, the output of that power supply goes to the 5V USB input of your camera setup.

The downside is that it will reboot the camera every time someone rings the doorbell, because you are shorting across the camera's power supply.

You can put a resistor in series with the button to fix this. You will need to find a resistance that's low enough to still cause the chime to ring, but high enough not to disrupt the camera's power supply. Maybe start around 20 ohms. If you can't find a working resistor value, you can change the transformer to a 24V or 36V transformer, but make sure to keep that resistor high enough not to burn out the chime, and make sure your 5VDC power supply can handle the increase in input voltage.

Back when I worked at IBM, there were a bunch of flags hanging in the cafeteria that represented every country where IBM did business. We often wondered, why wasn't there a Nazi Germany flag? After all, IBM did sell a ton of machines to the Nazis to keep track of Jews and other undesirables, in order to commit genocide. I wonder why IBM wouldn't want people to know about that? /s

https://en.wikipedia.org/wiki/IBM_and_World_War_II