R740, changed to H730p Mini - Checking DP/N's because of disk slot number oddness

Hello!

Fresh start with cleared disks, no foreign config. (Tried Intel 1xS4500, 1xS4510, 4xSamsung P863's).

R740 with 8x SFF backplane (0R3GPC v 4.35). Bios 2.19.1. 

Removed H330 (099t5j). Removed backplane data cable (09mhyn).

Added H730P Mini (0YRPP6) (fw 25.5.9.0001)

Added Interposer (004M4C)

Added backplane data cable (0N7HT1)

Experiencing unexpected behaviour with the indexing of the physical disks.

Physical Disk Management for instance is reporting:

Physical Disk --:--:05: SSD, SATA, 1.745TB, READY (512B) for the disk that is physically located in slot 00:01:01

Slot 01:00 seems to register fine, but the others vary depending on the disks installed. It's driving me nuts trying to track it down. Have I done anything obviously dumb?

This makes me wonder if I've grabbed the wrong backplane cable perhaps? Or any other quirk that might be obvious to someone who knows, but I've clearly missed? Input and ideas gratefully received, thanks!