What suddenly reformatted my external backup hard drive?

17,1 imac, macos 10.13.1, fusion drive w time machine backup external thunderbolt connected 1tb hfs+ external hard drive.

updated high sierra on imac fusion drive couple weeks ago, not apfs. hs time machine continued backup fusion drive external 1tb hfs+ hard drive until last night when message “the time machine drive no longer available backup.”

using hs disk utility, mounted 1tb external backup drive. had reformatted 1tb backup drive apfs, , incompatible backing main high sierra fusion hfs+ drive.

did not instruct imac reformat external hard drive. did instruction reformat apfs external drive come from? why system decide format external backup apfs?

 

hard answer because there many possibilities.

might able find reason looking @ logs via console application. unfortunately apple "improved" way logs used in sierra , high sierra. may not easy find relevant information try searching console name of backup disk , see if find relevant info.

 

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