I'm sorry, but the implementation of the RestoreableDriveManager seems to be odd.

Here are some recommendations:

• Add names to the roles in DriveManager, currently they are not defined and juse just Qt::UserRole + 1 and Qt::UserRole + 2. To that add something like Restoreable role so this is accessible property. This would look like:

enum ItemRole {
   NameRole = Qt::DisplayRole,
   DriveRole = Qt::UserRole + 1,
   RestoreableRole
}

And do the needed adjustments in the DriveManager implementation

• Then the RestoreableDriveManager::filterAcceptsRow() can look like:

const QModelIndex index = sourceModel()->index(source_row, 0, source_parent);
const bool isRestoreable = sourceModel()->data(index, DriveManger::RestoreableRole).toBool();
return isRestoreable;

• I don't think you have to override QSortFilterProxyModel::roleNames() and QSortFilterProxyModel::data().
• And I think we can get rid of the logic with lastRestoreable or selected property that you just added and simply get the selected drive from the combobox using the DriveRole.