I had a spare NVMe SSD lying around (a Western Digital's SN850x) and plenty of USB-C cables. All I needed to do was pick an enclosure that let's the SSD connect to my PC through a USB port.

If you're upgrading your SSD, you can turn your old one into a fully functioning external storage drive for your devices using a storage enclosure ...

CNET editor Dong Ngo shows how you can replace your computer's hard drive with a solid-state drive and be significantly happier with your life.

Plus you’ll likely save a lot of money by DIYing your own external SSD. Large-capacity PCIe drives are available at surprisingly low prices.

There are few ways to make a laptop faster that are as easy and cost-effective as replacing your existing hard drive with a solid state drive (SSD).

So, instead of installing your SSD in the Optibay, you'll want to disconnect your regular hard drive, install the SSD in that slot, and then install your old drive in the Optibay and continue as ...

But if you’re going to have both the SSD and original hard drive in your Mac, you could pick a smaller SSD for OS X and your apps, keeping your music, photos, and videos on the old hard drive.