Copying files from PC to "external" SD card

I've got AirDroid and SSHDroid installed on a ZTE Axon 7 running 7.1.1. The phone has a micro SD card in it, and it can access (read from) that card successfully. What I *can't* do is copy files onto it. I can log in via ssh and see the file system, but I can't touch it. I've had a Moto X Pure for the past year, and via SSHDroid and AirDroid I can readily copy files to the SD card. Is this just a "security" (ie, "pay somebody") thing in the more-proprietary OS on the ZTE phone? Obviously I can mount the micro SD card directly on the computer, but juggling those tiny cards is kind-of a pain. If there are other solutions, I'd be grateful. *edit* — I've tried MTP with the Axon. I plug in the phone and tell it I want to connect as a "media device" with MTP. I then run mtpfs (I'm running Linux on the laptop; Ubuntu 16.04) and I get: Listing raw device(s) Device 0 (VID=19d2 and PID=0306) is a ZTE V790/Blade 3. Found 1 device(s): ZTE: V790/Blade 3 (19d2:0306) @ bus 2, dev 98 Attempting to connect device Android device detected, assigning default bug flags Then it just sits there. (I've never had much luck with MTP.) *edit again* — ADB seems to work. I don't move files that much (mostly music and audiobooks) and while ADB is a pain it's easier than fiddling with the cards.