Yes, the recent phones from HTC do got micro-sd slot which actually can be a pretty good thing.

But, just how do I know what card I should get since there is thousands of different ones?

I tend to say this: for a phone like One M8 you need a card that’s at least Class 10, a UHS-1 card is often more than enough. And, it also depends on your budget of course. Get as fast card as you can afford, even if you have to step down in size.

 

	Mark	Minimum Serial Data	SD Bus Mode	Application
UHS Speed Class		30MB/s	UHS-II UHS-I	4K2K Video Recording
		10MB/s		Full HD Video Recording HD Still Image Continuous Shooting
Speed Class		10MB/s	High Speed
		6MB/s	Normal Speed	HD and Full HD Video Recording
		4MB/s
		2MB/s		Standard Video Recording

Speed classes comes from HERE

Ok, now I know what class of card to get. But what brand?

I would recommend that get a card from a well-known manufacturer. Like: Sandisk, Kingston, Samsung, Lexar. There is of course many other makers around, always check reviews online if you decide to get a card from a brand that you’re not familiar with.

Google is your friend, with a little time and energy you can find the best card for your needs.

But, from where do I buy my new card?

That can sometimes be a little tricky, and sometimes depends on where you are. If possible: buy it from a large well-known retailer, either online or a real store. Avoid Ebay sellers if possible if they’re outside EU/US/UK/etc since you sometimes don’t get what you’ve ordered. Same as when it comes to the card itself, check the seller if you never dealt with them before.