MacBook Air Extra SD Card

Hello.

I'm on a MacBook Air - 2014 model, with OS X 10.11.4 installed.

Like every 13' inch model, my MacBook Air comes with a Memory Card reader. I was thinking of buying an SD card with a lot of Gbs... and also, thinking of connecting it 'permanently' to my MacBook Air. When I write permanently, what I mean is, it's is meant to stay connected to my Memory Card Slot 24 hours a day.

My question is: how safe is it, to use an SD card as a secondary 'SSD' card? Does it get more easily corrupted? Is it safe enough to use it a a Time Machine backup?! And if I decided to use it as a Time Machine backup, is it safer to remove it every single time I make a backup? Or could I simply just let it stay connected all the time?

Thanks in advance for any help you may provide me with.

Kind regards.