Ghosts? My Backup Volume is suddenly protected against writing - though I didn't set a passwort

WD External and Portable SSD External SSD for macOS
1

For 3 years, my time machine backup on WD SmartWare has worked fine. The last backup was automatically done on April 13th, 2016. Instead of today’s backup, I receive the following error message: “Es ist nicht möglich, Dateien auf das Backup-Volume zu kopieren, da dieses schreibgeschützt ist. Sie müssen das Volume u. U. mit dem Festplattendienstprogramm reparieren oder löschen. wenn das Volume nicht repariert werden kann, wählen Sie ein anderes Volume für die Backups aus.” (short translation: Impossible to copy data onto backup volume because it is protected. You must eventually repair oder erase the volume …).
I have not done anything between April 13th and today (no updates, no electricity blackout). And the check with WD Drive Utilities didn’t reveal anything (all fine).
Does anybody have an idea what’s wrong or what I can do (besides erasing all my backup data)?

2

Hello,

Have you tried running first aid on the disk utilities?

3

First aid? Where’s that? I find three levels of tests on the WD Drive Utilities (done them all, they’re all ok), but nothing called “First Aid”.