I suspect the SSD I have installed was recognized as a hybrid drive when I did the original High Sierra upgrade, so it left it as HFS+ instead of migrating to APFS. I want to load the 500gig with the new High Sierra and I want to do this before it is fitted in the 21 1/2. I am trying to do a clean install on the 21 1/2 and change the 2tb SSHD out to be replaced by a 500gig spinny disk. Everything seems to be working ok and hopefully now it will move on normally. iMac 12 1/2 and MacBook Pro 13, both running High Sierra and the new iMac M1 running Big Sur. My machine now shows that it is on 10.13.3 and that the SSD is formatted as APFS.
In disk utility, I had to erase my internal SSD and format it with APFS.boot from the USB and enter disk utility from the recovery options menu.Thus, it boosts performance and helps manage.
CLEAN INSTALL MAC OS SIERRA IMAC 2011 PDF
Improves VoiceOver navigation when viewing PDF documents in Preview. This update: Improves compatibility with certain third-party USB audio devices.
CLEAN INSTALL MAC OS SIERRA IMAC 2011 UPDATE
It enables the storage and encryption of a large number of files. The macOS High Sierra 10.13.2 update improves the stability, compatibility and security of your Mac, and is recommended for all users. The Apple file system is the most significant feature it brought. There are few bells and whistles, but the changes are noticeable and positive.
We shall see if it takes future updates okay. So.I was ultimately able to get my machine to 10.13.3.