Expanding HDDs

[Kaizagade]

Hey, i have 3 x 4TB HDDs in a pool. I did this so i had the option of expanding it in the future, I have just went and added 2 x 500GB HDDs to my server and it wont show them when i try to extend the pool. It says "You need to have a suitable drive candidate available to expand your pool. Check out our article on selecting drives to better understand what is required."  and tells me to "read here" and said page has no useful information regarding this. Where can i read up about all these annoying little things about what can be expanded and how. As if i knew this as the case i wouldnt have bothered with a third drive to be able to expand in the future if i cant expand it easily. - Thanks