Speaking of the Portal server specifically, when we installed Portal (v10.5.1), it went into C:\Program Files\ArcGIS\Portal.
After creating our portal, there was also the C:\arcgisportal, where the content apparently goes.
I was able to use the webgisdr tool (C:\Program Files\ArcGIS\Portal\tools), to successfully make a backup of our Portal system.
Later, we had a failure and had to restore from the webgisdr backup. Unfortunately, the Datastore and ArcGIS Server backups restored fine, but the Portal part did not.
Since the content went into C:\arcgisportal, it seemed that all IT had to backup from their side, was this folder. Our IT folks do not usually do full disk images on servers. During the troubleshooting, I learned that the C:\arcgisportal folder is insufficient to restore from (for just the Portal part).
What must we have our IT folks backup, so that we can restore the enough of the disk, to come back up, if the webgisdr backup has a problem?
