That's possible, but really challenging to do because mass storage is block oriented, the host (desktop) is likely to do read and write caching, and there's no mechanism for a host to say I finished writing this file, and there's no mechanism for a device (phone) to say blocks changed from under you.

The paradigm for block oriented filesystem access is exclusive access, and filesystem code is built around that. There's some niche filesystems around multiple simultaneous access to block devices, but I don't know if there's any that are open source; mostly people don't setup scsi/sas/das disk arrays with two hosts anymore, and when they do, they're much more likely to have exclusive access with failover than simultaneous access.

If you had a team of really detail oriented developers capable of getting this done for Android and desktop platforms, wouldn't you rather they work on something else?

Another approach might be to build a virtual filesystem to export as a block device on usb connection that's essentially a snapshot of the current one, and then you sync any changes that were written on usb disconnect, but then you need to manage divergent changes and that's unfun too.

SMB over USB would be terrible in many ways, but probably handle this use case much better.

> If you had a team of really detail oriented developers capable of getting this done for Android and desktop platforms, wouldn't you rather they work on something else?

If you can improve the world (mass storage device is really wide spread) this way, why not?

For better or worse, most people's photos never get transferred to a computer. Heck, there's a ton of people who don't have a non-phone computing device. Pushing their photos to the cloud so they see them after their current device dies is better than working on a new filesystem that allows for multiple host simultaneous access and porting that to everything. I could make a huge list of more tractable things that this hypothetical team could work on to make Android better for way more people.

Top on my list would be getting it so the touch screen just works every time. I can't count the number of times I've had to turn the screen off and on, because the touch screen came up in a way that I can't swipe up to get the code entry because swiping from the bottom to the top of the screen doesn't move it enough. I've had this happen on pretty much all my androids.

Things like booting faster would actually be nice. Especially since sometimes phones reboot in pockets. Setting up applications for faster starting would be amazing. It's not all in Google's court, but a basic hello world with the Android IDE starts up rather slow, even if you've noticed you need to compile a release build.