After installing the recent update to Safari on my iMac, suddenly Time Machine gave me errors that my Time Machine drive is read only. WTF? I checked the permission, it said I can read & write, but on the Get Info pane of the Time Machine drive still said it's read only. I restarted my iMac, no difference. I fired up disk utility, did a verify disk and canceled it (because it's taking too long), and suddenly Time Machine is okay. I restarted my iMac again, and Time Machine gave the error again. Finally, I ejected the drive, unplugged its cable, and reconnected it, and everything is fine so far. Weird.
This is an issue with MacOS in general. Most of the time, the OS is great without issues, but then there are times when weird things like this happened without a definitive solution/answer. I googled the issue and various people pretty much tried all things that I did above as trial & error in solving the problem.