OK.... figured it out. I've been wanting to understand this myself.
The plug ends of SATA and SAS drives are very much alike, and there is a "key", a plastic projection that is present on the SATA drive connectors between the SATA data and the SATA power parts of the drive's plug. A receiver for this drive plug can be designed to accept a SAS drive but reject a SATA drive by physically blocking this key, which is not present on a SAS drive plug. Alternatively, a receiver for the drive plug end can be designed to be "universal", accepting both SAS and SATA drives. Here is a picture of what the drive plug ends looks like, which you can see when a moderator releases it. The yellow circle highlights the "key" present on SATA drives:
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HP has made some quality drive adapters that many of us are using, for converting 2.5" form factor SSDs into 3.5" form factor units that then work perfectly in the Z600/Z800/Z620/Z820 drive sleds you mention. Both 2.5" SATA SSDs and HDDs, and also SAS 2.5" drives, fit in those drive sleds once they are attached to these "universal" HP adapters (which shift the plug that meshes with the Blind Mate receiver for data/power connections way back in the drive drawer bay).
KEY INFO: You do not need to use that gray plastic adapter on a 3.5" SAS drive to have it work perfectly as designed for the drive sleds in these workstations. This is because the "Blind Mate" receiver built into the rear of each drive sled bays is a "universal" receiver, accepting both SATA and SAS drives as is. You do need that gray plastic adapter if using a 3.5" SAS drive in a 5.25" bay adapter or in a more basic workstation such as the Z400 which does not use the Blind Mate receivers. The HP 2.5" to 3.5" form factor adapters shown below take the place of that gray plastic adapter any time they are used. Their receiver is "universal" and the cheapest one originally was designed by HP (made by Foxconn) for 2.5" form factor SAS drives to be attached to 3.5" form factor server drive sleds.
This cheaper one can be seen on eBay via search for 654540-001 or 654540-002, and has a universal receiver that is shown nicely in the image below. Note there is no plastic projection blocking that gap between the data and the power portions of the receiver (a "Blind Mate" receiver where you can see it). It will accept SAS and SATA 2.5" drives or SSDs equally well. This is the one I use and recommend over all others now:
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The more expensive version is "tool-less", and has part number 668261-001. There also is a Western Digital IcePack that was designed for the 2.5" SAS and SATA high RPM Raptor HDDs, and is universal too. You can see that on eBay but those are unnecessarily expensive. Here's the fancy HP tool-less version, also generally too expensive, and it uses the exact same Blind Mate receiver and quality of metal and fabrication as the cheaper one above:
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Now all you have to do is remove that gray adapter, slide your 3.5" SAS drive into the drawer bay that has 0 stamped in the metal next to it, and go find the SATA cable running from the Blind Mate connector back in there down to the SATA port 0 (far right) and switch it down to the SAS port on the far right too.
Personally, I'd recommend SSD much more highly.