See HP page, your Elitebook 820 g2 has 2 slots, 1 for an m.2 SSD une 1 for a 2.5 SSD like this Crucial MX500 up to 1 To. So you can buy that. I have 3 Crucial MX500 in my two Elitebook (M.2 512 Go, 2.5 512 Go and 1 To) and I am satisfied.

https://h30434.www3.hp.com/t5...

Ich wusste nicht dass man kan auch 2 To MX500 verbauen

https://www.crucial.com/usa...

I must say that if you see that an Elitebook can have 1 To HD, you can also install a 1 To SATA (not PCie) SSD. When the Elitebook 820 G2 came out, there were no HP Sata 1 To.