hp elite 8300 usdt miniPCIe port

[jerrys01]

Can I use a nVME drive in the miniPCIe port?  If so, is it bootable?

[SDH]

I'm going to say no.

Look at the differences between the two slot pinouts:

HERE 

AND HERE 

Beyond the physical incompatibility issue the Elite series family is a business class set of boxes roughly from the ZX20 era of workstations.  The Z Turbo Drive generation 1 works in the ZX20 workstations but uses an AHCI based controller stick.  It is not until you get to the next generation of workstations, the ZX40 family, that you see the Z Turbo Drive generation 2 (NVMe controller based stick) be able to work.

Then there is the BIOS issue... overall just not possible.

However, your box does have SATAIII technology so making sure you have a high quality SATAIII SSD installed is a wise move.  For the other larger 3 members of the Elite 8300 family that use 3.5" form factor drives you can find very high quality 3.5" to 2.5" HP adapters for about 9.00 on eBay... search for 654540-001 or 654540-002 there.

Your USDT version is the smallest in the family and has one 2.5" form factor internal drive bay so you don't need an adapter to run a 2.5" SSD inside.