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Alternatives to MTP for Android

MTP seems to be broken on my computers, I can't transfer files with Opus to/from my phone.

The recommended alternative is FTP, but it's extremely slow due to going over wifi. It also doesn't work with wifi device isolation, or LAN segregation.

What other options exist? I've tried using a USB ethernet adapter and it's fast but rapidly drains the battery.

You could use a cloud service and your phone's mobile radio, assuming your data plan is beefy enough. A good client is Solid Explorer, which directly supports several cloud connections (the usual suspects plus a few more).