Question: Time Machine Erroneously Says Next backup "When disk is connected"
Since upgrading a MacBook Pro and an iMac to High Sierra (10.13.3), I have noticed the following anomaly with Time Machine.
1. If I access TM from the Menu Bar icon ("Open Time Machine Preferences"), TM reports "Next backup: When disk is connected", even though a disk is already connected (and accessible through Finder). [If I then un-check the "Back Up Automatically" box, and then immediately re-check it, the next backup time is correctly displayed; however if the window is closed, and then re-opened from the Menu Bar, the original erroneous message is again displayed.]
2. But if TM is accessed through the System Preferences panel, it correctly reports the time of the next backup.
TM is otherwise functioning correctly; but you'd expect the TM preference screen to show the same result, whether it was accessed via methods #1 and #2 above. But it doesn't.
My MacBook Pro is using a 1 TB Seagate Backup Plus external drive; the iMac is using a 2 TB WD Elements external drive. Both external drives are partitioned to allocate 75% of their space to Time Machine, and 25% of their space for other purposes.
This issue did not appear prior to my upgrading from Sierra to High Sierra. Given that it appears on two different Mac models, and two different manufacturers' external drives, my sense is that this is a High Sierra issue.