Mac Pro 2012 upgrade hard drive

One point is the sled SATA connections are only SATA II which is slower to the present SATA III drive. Thus you will not gain that much by placing a SATA II SSD in a sled SATA connections.

First step up in speed would be to use a PCie card that accepts an 2.5" SSD. Here is one I used.

https://www.amazon.com/OWC-Accelsior-PCIe-Adapter-Drives/dp/B00WUZPMHE/ref=pd_day0_3_sccl_14/140-7519353-4632920?pd_rd_w=4J8dX&pf_rd_p=8ca997d7-1ea0-4c8f-9e14-a6d756b83e30&pf_rd_r=Q93Q7HN46WDZASSY4GEF&pd_rd_r=55d5bad1-2e02-4af0-b043-09822f467b7f&pd_rd_wg=ik8Xh&pd_rd_i=B00WUZPMHE&th=1

I was and may still be available in a card that accept two, 2.5" SSDs.

Fasted increase in speed would be to use a PCie card that accept M.2 NVMe blade SSD. He is one that I think I used:

https://www.amazon.com/M-2-Adapter-Aluminum-Heatsink-Solution/dp/B07JJTVGZM/ref=sr_1_3?keywords=pcie+nvme+card&qid=1651068364&sr=8-3:

To use the NVMe SSD you need a fully upgraded MP 5.1 (or 4.1 flashed with 5.1 firmware)

I used to have a 4.1 flashed MP and it worked great with the PCie NVMe SSD, very fast. The power supply died and I gave up on it.

A really less expensive adapter for a standard sled is:

https://www.amazon.com/ORICO-Adapter-Mounting-Bracket-Interface/dp/B01LZWX6PD/ref=pd_rhf_dp_s_all_spx_wp_sccl_9/140-7519353-4632920?pd_rd_w=clpc1&pf_rd_p=cfcbf4ee-c7ed-4129-9e85-19f540a08162&pf_rd_r=9R621XR8G68S8XDGZ9H5&pd_rd_r=b9edb6ad-32ab-4cc7-8ec1-0d9a1aaaf0cd&pd_rd_wg=McAWw&pd_rd_i=B01LZWX6PD&psc=1

An ordinary 2.5-in SSD for the boot drive is a big improvement, and using a replacement sled or a sled adapter could not be simpler.

There is also the "front drive free" hack. If you manually place the SSD drive in bay #1 at the front, and put a small block of wood (about 3/4 in thick) under it and on top of the intake fan, it works with no sled at all.

Once you get a SATA card in a Bay working, you can consider going faster still using a PCIe slot card.

Using a PCIe slot card is somewhat more complex and has more pitfalls along the way. Read the claimed top speeds with EXTREME caution, because many of the touted speeds can only be reached in the 2019 model Mac Pro with much faster PCIe bus.

Most brand-name SATA SSD drives from major makers will work.

If you are in the US, a wide assortment of single and multiple drives and external enclosures appropriate for your exact model Mac can be found at Mac-centric Illinois, USA Vendor OWC. They pride themselves on their Mac support, so anything they recommend works with your Mac AND is trouble-free.

If you are not in the US, you can still use their site and recommendations as a guide for purchasing in your market.

https://eshop.macsales.com

Use the "My Upgrades" link at the center top of the page and specify your Mac to get recommendation for ONLY your model Mac (and not a lot of "chaff").

Their drives are "pretty good", as are their prices. Sure, you may be able to do better with careful, focused shopping, but you can also end up with stuff that won't work in your Mac.

Get an adopter like one of these

https://www.amazon.com/ICY-DOCK-Converter-Mount-Adapter/dp/B002Z2QDNE?th=1

I have used one in similar slide-in sled.

This works as well, and they finally cut the price to a much more reasonable US$14.75:

https://eshop.macsales.com/item/OWC/MMP35T25/

( But does not fit the 2006 through 2008 models.