We've been using SATA (ADATA or Transcend 2,5" SSD's), but a much neater solution would be to use a PCIe SSD (M.2 SSD + PCIe adapter).
We've tried M.2 WD Green SSD's as wel as M.2 Kingston SSD's. Both to no avail. The interesting fact is, when I use an M.2 -> Sata adapter and an Sata -> USB adapter, it can actually boot fine.
When I use the above configuration (M.2->Sata->USB) boot-repair-disk has the option to repair the boot, but with the M.2->PCIe I do not get this option. Also see (http://paste.ubuntu.com/p/jpKWMP8RMY/).
Before I throw this idea into the bin, I would like to know if the HP DL360 Gen9 has any chance of booting from a PCIe SSD and possibly why not.
Adapter used (they look oddly similiar):
- Lindy M.2 SSD to PCIe Adapter Card
- StarTech x4 PCI Express to M.2 PCIe SSD adapter