I installed a bulk 3.5 inch HDD in HP Z1 G9 Tower Desktop.

[ikayos]

If you purchase the Z1 G9 Tower Desktop without HDDs, it does not come with jigs for installing a bulk 3.5-inch HDD. I was able to install it by using the following screws.
Product number: 450712-001
Step 1: Attach the 4 screws to the bulk HDD. The screw holes are a special star shape, so you will need a T15 screwdriver.
Step 2: Insert the HDD into the 3.5-inch bay until you hear a click and it is secured in place.
Step 3: Attach the power cable and communication cable. The communication cable is not included.
Good luck!

[SDH]

The silver headed screw ones have Imperial 6-32 threads and the ones with black headed screws have M3 metric threads. Both use the same Torx driver.

3.5" form factor devices use the silver headed screws. Optical drives and 2.5" form factor HDD and SSDs use the black headed ones. Here's a pic of the M3 black ones, much harder to find. The blue part is an elastomeric vibration dampener material:

M3 metric threads

[SDH]

Another related high quality HP item worth knowing about is the special 2.5 to 3.5 inch form factor adapter that allows you to put a 2.5" drive into an internal 3.5" space. The eBay HP part number to look for is 654540-001 or 654540-002. 

For your use in those slider slots you'd want 4 of the silver headed type shown above (#6-32) and to attach the 2.5" drive inside the adapter you also need 4 short wide headed M3 metric screws (which often come with the eBay purchase). Under 10.00 USD each... 

3.5" to 2.5" HP adapter

EDIT: Another significant benefit of these HP adapters is that they effectively shift the 2.5" form factor's power/data connectors to the left (viewed from drive "rear") so that the net result perfectly matches a standard 3.5" drive. This means you can perfectly retrofit a 2.5" SSD into the Z6 series and Z8 series drive bays that use the "Blindmate" receivers built into the depth of the drive bay for mounting a drive/adapter in the caddy "drawers" that are made to mesh inside those bays, shown below. The adapters are made by Foxconn for HP in China, A+++ build quality:

Adapter works with these drive bay ''drawers'' perfectly