The HP t420 came in a variety of configurations including Windows Embedded Standard 7E. It looks like WES7E vanished as an option 2015/2016 leaving just ThinPro and Smart Zero Core as the shipping OSs. There are embedded controls that prevent you installing the HP tailored WES7 OS on any t420 that was not originally licensed for it. If you want to run Windows 7 on your t420 your route is to fall back to the standard Windows 7 install ISO. As is often the case things aren't always that simple....
In August 2022 I heard from April who had worked out a way of adding a M.2 SSD to the t420.
Background
As you likely know, the HP t420 is intended by HP to boot from USB or the network, and... nothing else. For Linux this is of course fine, and Windows 10 can be convinced to work with this without too much effort too. But for Windows 7, which is arguably the best-suited version of Windows for the t420, it's not so simple (see on) and this got me looking for ways to get a proper SSD connected.
FYI: By default the Windows 7 installer will not install to a USB device. If you work round this and get it installed you will find it will not create a swap file on anything it thinks is a removable disk. Work around this and then.....
Solution
This led me to the other connector inside the t420, the mini-PCIe slot intended for the WLAN card (no such card was fitted in my unit). After a bit of searching I found an adapter that would at least make the physical connection - it was being sold as M.2 Key M (NGFF) NVME SSD To Mini PCIe Adapter Card For WIN10 & Cable Supply USA on eBay, but as eBay listings tend to do, these will likely come and go and vary depending on country as well. The adapter itself consists of two PCBs - one fits in the mini-PCIe slot, and the other has a socket for an M.2 SSD. It came with a ribbon cable that connects the two together.
[David] In a quick check I did not find anything on eBay.co.uk but it did pop up on amazon.com
and amazon.co.uk.
With the drive physically connected, this is enough for an OS to see it (for example, you could boot from USB and then store data on the SSD), but to boot from the SSD, firmware support at power-on is required - and the firmware in the t420 doesn't support this, as it was never intended by HP to be an offered feature.
However, some SSDs, mostly the earliest NVMe models, work around this by providing an option ROM to effectively "inject" the necessary firmware support. I had a Samsung 950 Pro here which has one, so I put that in, and sure enough it worked, and I was able to install Windows 7 (after loading the proper drivers).
Possible hardware
The SSDs that I believe contain option ROMs are:
- Samsung 950 Pro (NVMe, tested working)
- Kingston HyperX Predator (AHCI, meaning it pretends to be a SATA host with a drive connected - this might be the one of choice for Windows 7 as it won't need a driver)
- Samsung PM963 (NVMe)
- Plextor M8pe (NVMe)
- Plextor M8se (NVMe)
- Intel 750 Series (NVMe, but not M.2, would need a different adapter and possibly external power)