The initial goal and converting APFS to HFS+
Actually this is old news as it was brought up in several Apple warnings about APFS. Still good as a reminder since converting a Time Machine drive to APFS very nicely destroys the LINKs that Time Machine uses to store file data. Convert HFS to APFS 5m 10s. Create an APFS disk image 2m 54s. Create an APFS container and volume 7m 3s. Running a file system check 3m 46s. You can convert to APFS in Disk Utility. Just right click on the name of your disk, and it'll do the conversion. I do strongly recommend you make sure you have good backups when you do this, although this is supposed to be a non-destructive operation.
I wanted to use Time machine for Mac OS backups for the first time. Time Machine backup does not currently work with a partition formatted with the new APFS format from Apple. The external drive and partition I wanted to use for Time Machine backups had a partition formatted as APFS because I had used it to boot Mac OS. The solution was to convert it to HFS+ (also known as Mac OS Extended). This is a destructive conversion but I no longer needed the drive for booting Mac OS on an external drive.
I used the instructions here for converting the partition to HFS+. This basically involves 'erasing' the partition (and selecting HFS+ or Mac OS Extended) with Disk Utility in Mac OS. After the conversion, Time Machine worked fine for backing up to that partition on my external hard drive.
Convert Hfs To Apfs Greyed Out
Unfortunate side effect
The unfortunate side effect was that the exFat partition I had on the external drive could no longer be seen by Windows for reading/writing.
The behavior was strange. Disk Manager in Windows could see the drive and the partition but couldn't do anything with that partition (and no drive letter was assigned). When I right clicked that partition (in disk manager), and selected 'Change drive letter' or 'Properties', nothing happened.
What went wrong and the solution
Apparently, somewhere along the way of converting the APFS partition to HFS+, the exFAT partition became inactive. The easy fix was to make the exFat partition active. I used MiniTool Partition Wizard (free version) in Windows to mark it active. Then ejected the drive, unplugged it and plugged it back in, and it suddenly was seen by Windows (10).
Apfs To Ntfs Converter
See other posts on Mac OS:
Convert Hfs To Apfs
The external drive I was using is a Lacie Rugged Thunderbolt USB-C 2TB 130MB/S from Amazon
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