Like I said, it's something I learned the hard way. Many card readers do not write to the card. I had issues with 3 different brand card readers not writing, and had to talk to the manufacturer about it. Turns out, they said those readers do not support writing. That's why I mentioned to "double check" and make sure it can write.
I have dealt with numerous card readers over the last few years. More than 50, as I have installed then for quite a few people. I have also dealt with a large number of portable readers. While most of the portable USB stick style readers can also write, some cannot. Make sure it is advertised as being able to write as well as read. It doesn't have to say reader/writer in the product name, but it should say that it writes somewhere on the packaging. If not, it may not, and your mileage may vary. I was burned more than once by this, and never again.
I have also noticed card reader/writers are a little more expensive in some cases than just plain readers, but not by much, and it depends on the brand.
My wife has a card reader. It will not write. It will not format. I have a card reader/writer, and it does it all.