Confused about why I cannot RDM disk - what are the requirements?

[AveryFreeman]

Hello,

ESXi user since 5.5, but new to RDM disks.  I had always passed through an entire LSI 2008 controller in the past in order to operate a VM with ZFS disk array.

I recently installed a Microsemi SmartHBA and was able to migrate two RDM disks I had passed through to a Windows Server 2019 VM using vmkfstools -z /vmfs/devices/disks/xxx /vmfs/volumes/path/to/vm/rawdisk.vmdk to the VM using the GUI -> New RDM disk once I plugged the two disks into the new controller.

Now I have 2x 8TB HGST drives I'd like to use with another VM as RDM disks, so I plugged them into the same controller, but the option is not available.  Same controller, same host, different drives.

Which begs the question:  What are the requirements for a disk to be available via RDM?  I can't seem to find anything specific in the KB articles - just that RDM is possible, and the various implications of using it vs. virtual disks, etc.

I've tried removing all the SmartHBA's controller caching features.  It is set as a simple HBA on the entire 4-device port. Two other drives are working fine as supported RDM disks in the same host on the same very port.

Here is the output of two hard drives to show contrast.  #1 is the disk I am trying to use as RDM, but cannot - it is a SAS3 HGST He8 8TB 4Kn drive:

naa.5000cca261018fc0
   Device Type: Direct-Access
   Size: 7630885 MB
   Display Name: Local HGST Disk (naa.5000cca261018fc0)
   Multipath Plugin: NMP
   Console Device: /vmfs/devices/disks/naa.5000cca261018fc0
   Devfs Path: /vmfs/devices/disks/naa.5000cca261018fc0
   Vendor: HGST      Model: H5H72808CLAR8000  Revis: M9L0
   SCSI Level: 6  Is Pseudo: false Status: on
   Is RDM Capable: false Is Removable: false
   Is Local: true  Is SSD: false
   Other Names:
      vml.02000000005000cca261018fc0483548373238
   VAAI Status: unsupported

 #2 is a 2TB WDGreen 4Kn SATA2 drive:

   Device Type: Direct-Access
   Size: 1907729 MB
   Display Name: Local ATA Disk (mpx.vmhba3:C0:T65:L0)
   Multipath Plugin: NMP
   Console Device: /vmfs/devices/disks/mpx.vmhba3:C0:T65:L0
   Devfs Path: /vmfs/devices/disks/mpx.vmhba3:C0:T65:L0
   Vendor: ATA       Model: WDC WD20EZRX-00D  Revis: 0A80
   SCSI Level: 6  Is Pseudo: false Status: on
   Is RDM Capable: true  Is Removable: false
   Is Local: true  Is SSD: false
   Other Names:
      vml.020000000030000d170484f782574443205744
   VAAI Status: unsupported

Why is the SATA2 drive "rdm capable" while the SAS3 drive is not?  How can I know whether or not drives are "rdm capable" before I buy them?  I just bought these drives so I'm rather irritated that they're not working, had I known I could not use these drives for rdm I would have looked for something else.

Thanks