Sharing PostgreSQL database on one dual-boot machine without duplication (cross-platform)
I would like to know if it is possible to share the structure and data between the two systems without duplicating and/or having to copy everything each time I load a new OS. Effectively, I would like all the data to be available to Ubuntu when it is created in Windows, and vice versa.


You can't. Just not supported, not possible. No can do. PostgreSQL does not attempt to make the data directory portable across architectures, operating systems, etc.

The only way you can do this is to run a Windows virtual machine inside Ubuntu when you want to access a Windows data directory, or a Linux virtual machine inside Windows when you want to access a Linux data directory.

You might want to run a VM image that's shared between both platforms, and have it contain the database - but be careful, Linux's NTFS driver isn't great for performance, and might not have all the wrinkles ironed out when it comes to concurrent I/O so I'd be a little leery running something like a VM image off it. FAT32 and exFAT are not crash-safe. So there aren't really any good choices for a file system both systems can share to run "real work" off.

So personally... I'd just have a small separate machine. Or I'd keep a dump of the DB and reload it when I rebooted.

(Additionally, if running PostgreSQL under Linux on Linux's NTFS file system driver works, that's a little surprising, and I certainly wouldn't rely on it for anything I cared about).