It works! Some phones are just bad.
I got one of these phones too and had the same problem, even with a smaller/slower memory card. I contacted Motorola support via chat and they claimed the pending system update to 4.4.4 may help, but could not say when I could expect it, and offered to send a 4 gig memory card in the meantime. The support person said the update depended on my carrier, but I bought it unlocked from Amazon and use a BYOD plan, which seemed to flummox them. I waited until the end of the return window, still no update available, and had Amazon send out a replacement, which was free and speedy and they gave me 30 days to send the old one back. It works fine with the same memory card:
SanDisk Ultra 32GB MicroSDHC Class 10 UHS Memory Card Speed Up To 30MB/s With Adapter, Frustration-Free Packaging - SDSDQU-032G-AFFP-A
and seems to have the same system version, 4.4.3. The one difference is that I put files on the card first, which some forum posters seem to think makes a difference, but the Motorola support person said it would not. I will try wiping it sometime soon to see if the replacement phone can still read it, but it looks more and more to me like some of these phones ship with this problem and some do not. I recommend buying it from Amazon and just returning it until you get one that works as it should.
Now it is just the phone I was looking for: compact, fast, good battery life, expandable memory, minimal/nonexistent carrier software bloat.