Here is the general idea of the issue – with vSAN 6.7~ you cannot mix SAS / SATA Drives when using a P420i Raid controller with a DL360 Gen8.
I first thought it was vSAN only, but I also could not create a regular VMFS 5/6 datastore on the SATA Volumes (SAS worked fine for VMFS and for vSAN)
The Error I got from VMware vSphere was very generic, so here it is:
A general system error occurred: Failed to reserve disk naa.5002538da03397d4 with exception: Failed to reserve disk naa.5002538da03397d4 with exception: Reserve failed with error code: -1
Operation failed, diagnostics report: Unable to create Filesystem, please see VMkernel log for more details: Failed to create VMFS on device naa.5002538da03397…
I got around this by installing an H240 HBA controller that doesnt have any issues, and it appears on the VMware HCL.