Install Shelfroll
Two apps: Shelfroll Hub for your Mac and Shelfroll for your iPhone.
Not in the App Store yet
Shelfroll is being prepared for release. Store links will appear here when the apps are published.
1. On your Mac: Shelfroll Hub
Install it from the Mac App Store, open it, and choose your exFAT drive.
2. On each iPhone: Shelfroll
Install it from the App Store, open it, and scan the QR code shown by the Hub.
The Hub is needed for the iPhone app to do anything. If you opened this page from the iPhone app, finish the Mac step first.