I have a problem which I think is the same as Lucas, even though some of the details of my case are different. After going through many tests, reinstalls, etc., and talking with Apple and Promise support, my conclusion is some advance in Apple security has locked my Pegasus R4i, making it read-only. Apple support could not officially help me, because I am running 15.5.5 beta. Everything worked fine until beta 4, when the issue of not being able to use the R4i began.
I have confirmed several times that my drive is locked in such a way that I cannot unlock it.
R P, is there a way Lucas and I can test our drives to see if/how they are locked? And then unlock them?
I am very familiar with DiskWarrior, and have DiskWarrior 5.3.1 (the latest release). When I launch DW it evaluates the state of all HFS+ disks. (DW canot work on APFS formatted disks because Apple still has not released sufficient technical specifictions for APFS.) What DW does is build a completely new directory for the disk. DW presents a comparison between the old and new directories, then if the user consents replaces the old directory. This has long worked very well on HFS+ formatted disks.
DW tells me my R4i has an optimization index of 4, meaning the disks are very fragmented and would benefit from DW rebuilding.
When I click Rebuild, I get these error messages. My guess is DW cannot rebuild directories on a locked disk. 
