In September 2023 I heard from Carl who had added a second SSD to his 5070.
I've been running a Wyse 5070 as my home server for a few years.... I usually like to have a second disk in a mirrored config but up to now I've been running with only a single disk with the 5070. However I recently discovered that adapters exist to convert the A/E keyed PCIe Wifi slot to an NVME M keyed SSD slot.
For those not up-to-date with the M.2 interface it supports a wide range of signals/protocols on the connector. Exactly what might be offered on what pins is signalled by the position of the notch on the edge connector.
SSDs use the M keyID which supports PCIe x4 and SATA interfaces. (The latest NVMe SSDs interface over PCIe, older SSDs use SATA). The thin client hardware will either support NVMe or SATA. I'm not aware of any that support both. As noted on the hardware page the 5070 supports M.2 SATA SSDs.
The A and E variants of the M.2 socket are there for interfacing a range of other I/O devices. A popular use for these are for providing WiFi (via PCIe) and Bluetooth (via USB). The adaptor carries M.2 'M' socket which is cross connected to the PCIe channel on the A+E socket on the motherboard.
Carl sourced his hardware from Amazon. He used a 'Sintech M.2 NVME SSD to M2 A/E Key WiFi Port with 20cm Cable (M.2 Only Provide PCIe 1X Lane)' paired with a HP EX900 Plus 2TB NVMe PCIe M.2 Interface SSD, GEN 3 x 4, 8 Gb/s, 2280 3D NAND PC Internal Solid State Hard Drive Up to 3150 MB/s. (I've used the descriptors as any link may vanish at some point in the future).
Note that, whilst the standard SSD fitted to the 5070 is SATA based, this additional one, as it uses the PCIe interface, is NVMe. Also note that the standard 'M' option has PCIe x4 connection whilst the A+E interface only has PCIe x1. So the adapter (as noted in the product title) only offers a single PCIe channel for the SSD interface rather than the four lanes on a full 'M' interface so performance may be not be as good as it might be.
Sintech Adapter
Fitted in 5070