Adding a second SSD to Optiplex 3020

Yes as long as it's an MT or SFF.  Use double SSD adapter in an SFF.

Do you need any more details or are you good to go?

I think mine is an SFF judging by the photo you included. Can you point me to a guide on how to do this? I'm no expert.

It's the same as adding the first one, but you'll need the aforementioned double SSD adapter on Amazon.  It goes inside of the blue caddy.  And needed is SATA data cable. 

Installing and removing boot drives will be in the Optiplex 3020 SFF Owner's Manual online.  The double SSD adapter will likely also come with instructions.

Yes, it is possible to add a second SSD to the Optiplex 3020. The Optiplex 3020 typically comes with multiple drive bays, allowing you to install additional SSDs or hard drives.

3020 MT has 2 HDD bays.  SFF has 1.  Tech Guide attached below.

Good point @Chino de Oro .

Also, a SATA port PCIe card can add SATA ports.

I thought to mention that the motherboard only has two SATA ports. 

One SATA 3 for hard drive connection and one SATA 2 for optical drive connection.

If OP wants to add a second SSD/HDD drive, it will run at half of the speed of the first drive (SSD).

If a CD/DVD drive is already connected, there will be no SATA port left to use for second drive.

There is other option to look at.  If the PCIe x16 slot is not in use, it can be convert to storage use with faster speed of NVMe solid state drive via adapter.

If OP is interested, just ask.

I'm not technically qualified to do this, I think, although I have upgraded memory in another PC before. I'm happy to explore options. I've not opened the case, so I don't know if the PCIe x16 slot is in use. There is a CD/DVD drive I don't want to lose. I could replace the current 1TB SSD with a 2TB SSD - would that be the simplest solution?

That is a simple solution.  Specs say max capacity is 1TB.  But it's not unusual for users to exceed that in those models.

Here's the 'however' to the simple solution part - You'll have to either load a new 2TB SSD fresh or clone your old one.  I haven't cloned, so others can chime in on that one.  I do know it has to be done just right so PC doesn't end up seeing just 1TB.  I consider fresh Win10 load simpler and I do have instructions for it.  Whatever you want to do is up to you.

At first, I was under the impression you were going to use 1 SSD for boot drive and the other for storage.

Thanks for that advice. As this is not yet a vital problem - there's a fair bit of space left on my SSD - I'll come back to it in the future. If it ain't broke, don't fix it!

Speaking like a true Southerner.

Here is a very short video for adding SSD to PCIe slot.  Just to give you an idea.

I also believe in that some, "If it ain't broke..."  If the 3020 were an MT, you'd already have a 2nd HDD bay.  Insert SSD into an adapter; install in blue caddy; install blue caddy in HDD bay, plug in power and data cables and it's good to go.