Tails Installer should refuse upgrading a device that hasn't Tails installed
Originally created by Tails on #5623 (Redmine)
Apparently, some users try to use the “Upgrade from ISO” option to install Tails on a blank USB stick partition, and are surprised the resulting stick does not boot. Similarly, Tails Installer doesn’t prevent attempting “Clone & Upgrade” on a non-Tails device (at least if it has a GPT and a VFAT-formatted partition, and possibly in more cases).
Note that we want to add an “Install from ISO” option (#8865 (closed)). But regardless, Tails Installer should prevent users from trying non-working “upgrade” scenarios.
The list of potential target devices should be filtered to only display those with an existing Tails installation, when the user has chosen an upgrade scenario. It’s probably acceptable to only check for GPT + existence of a VFAT filesystem on a partition whose GPT label is “Tails”, so we don’t have to mount anything to check its content.
Feature Branch: kurono:feature/5623-Installer-should-refuse-empty-device