I wrote to iDrive Tech Support, and they say I should not be backing up files like "Programdata, Appdata, program files, caches, Library files etc."
Backing up individual photos and document files is nice, but I really need to backup my WindowsImageBackup backup set (which includes everything).
I think this is really a Microsoft Windows 8.1 problem: it is creating files under \WindowsImageBackup that is owned by different people.
Hence, no single 3rd-party application can read all those files.
The solution that works for me is:
Change iDrive service to use your user credentials.
\WIB manually (so you become the owner).
Share that directory, so you can access it via UNC (e.g.
wbAdmin start backup -backupTarget:\\MyPCName\WIB\ -include:C: -allCritical -user:MyUserName -password:MyPassword -quiet
to force it to create WindowsImageBackup under WIB, so that it will inherit the owner and permissions of the parent folder.
iDrive can now back that up just fine :)
You can verify permissions via a Windows PowerShell (run as Admin):
get-childitem D:\WIB -recurse | get-acl | Format-List