What suddenly reformatted my external backup hard drive?

[blackxacto]

17,1 iMac, macOS 10.13.1, FUSION drive w Time Machine to backup to an external Thunderbolt connected 1TB HFS+ external hard drive.

I updated to High Sierra on my iMac FUSION drive a couple weeks ago, not APFS. HS Time Machine continued to backup my fusion drive to an external 1TB HFS+ hard drive until last night when I get a message “The Time Machine drive is no longer available for backup.”

Using HS Disk Utility, I mounted the 1TB external backup drive. Something had reformatted the 1TB backup drive to APFS, and was now incompatible for backing up my main High Sierra FUSION HFS+ drive.

I DID NOT instruct my iMac to reformat the external hard drive. Where did the instruction to reformat to APFS the external drive come from? Why would the system decide to format the external backup to APFS?

 

[Ebenezum]

Hard to answer because there are too many possibilities.

You might be able to find the reason by looking at the logs via Console application. Unfortunately Apple "improved" the way logs are used in Sierra and High Sierra. It may not be easy to find the relevant information but you could try searching Console with the name of the backup disk and see if you find any relevant info.