Update to MacOS high Sierra not possible with Raid

when installing high sierra on a SSD, the volume will automatically be converted into a new disk format: APFS. that's what the installer in your case tried to do. unfortunately, afak this does not work with a RAID. and since your macbook is custom, the installer does not expect this scenario to happen - 2 SSDs in 1 macbook? sorry, cannot do...

so, what can you do?

high sierra should work on your RAID when you manage to keep its original HFS+ format. this document says that APFS is optional:

Prepare for APFS in macOS High Sierra - Apple Support

so try to work your way from there and find out how you can keep HFS+. sorry for not being of more help.

good luck!

you can install it on a USB Drive and the start from it, then create the RAID with the RAID assistent in the disk utility app, the n clone the USB with SuperDuper App to the RAID. now boot from the RAID with holding ALT at startup and set the startup disk to the RAID. but be aware all data on the drices will be erased when creating the RAID.

you can also clone your current OS on the RAID to a USB drive and update the USB to High Sierra an then clone it back with SuperDuper

This is not true. I'm trying to upgrade to High Sierra on a mirrored raid mechanical disk and I'm getting the same cannot install on apple raid message. It shouldn't be trying to a mechanical disk to APFS.

Mark

This is inaccurate, the Mac mini server comes in this config with two SATA drives so the installer DOES expect this scenario, in the usual way Apple are making decisions for us and not giving choice even to expert users.

NicW