So after some research, I think I've found the best method (with a pc) of getting files on and off of the Next core. This might be obvious, but I thought I'd write out the steps if it helps someone. I actually like the Mister core *better* than my old Spectrum Next N-Go clone if not only for the reason that I can now send things over to it via the network without having to physically move an SD card in and out of a machine.
1)Booting from a full install of the OS on the 2nd microsd, I renamed the used boot.vhd that lives in the ZXNext folder on the main Mister microsd, storage.vhd
2) Used Imdisk
https://sourceforge.net/projects/imdisk-toolkit/ to mount the image over samba.
3) Fully deleted the contents of storage.vhd from Windows, in order to use it as storage. Moved any games etc, over to the mounted storage.vhd (I have mine as the x: drive on my PC)
4) Booted the Next core from the microsd card, and then used the Mister menu to mount that storage.vhd in the ZXNext folder on the main Mister microsd as the D: drive. Use M to mount or U to dismount the storage.vhd and refresh any changes. press D to switch drives. All my files are listed on that storage.vhd for use. Profit!
Yes, I used the image while never dismounting it from Windows. But as long as I'm not modifying it's contents while the Next is running that vhd won't get corrupted. If it did, I guess I'd make another 10gb vhd and move that over. No biggie. If while using the Next, I want to move new content over or delete and have those changes show up on the D: drive, I make those changes from windows and then just press f4 to reset the Next and things get refreshed. I've also got an Ultimate 64 Elite FPGA in a breadbin case and I don't know why the Next guys didn't just build in an ftp server into the core like Gideon did for the ultimate. It was the one thing that frustrated me about it. And now that's solved with the Mister. Yeah, the Mister Next core doesn't like tzx files but honestly, I just boot into the regular Spectrum core for those, and I really don't miss anything. No biggie.
End of rant. Game on!