In December 2019 I heard from Jerzol from Poland who had added a SATA drive to his t620. You can read about that here.
In January 2021 I heard from Max from Italy who had added a VGA port to his t620 Plus. You can read about that here.
In February 2021 I heard from Pawel from Poland who had installed two 2.5" SATA drives into his t620. You can read about that here.
In May 2022 I heard from Evan from the US who had fitted a 2.5" SSD to his t620. You can read about that here.
In October 2023 I heard from Marco who had fitted a second ethernet interface to his t620. You can read about that here.
In February 2021 I heard from Pawel from Poland who had fitted two SATA 2.5" drives inside a standard t620 (note: Not the Plus). These were connected to a cheap mPCIe-to-SATA converter fitted in the mPCIe socket that's intended for a WLAN card. You need to have reasonable soldering skills to attempt this project. Here are his comments on the process.
Foreword: The early versions of the t620 have a mSATA socket fitted to the board. With the later board you have the option of soldering one in as the traces are still there, but mSATA drives are expensive, and it's easier to use the mPCIe socket as described here.
These drives are invisible to the BIOS and only appear once an OS with suitable drivers has been loaded. (Should be the case with most OSs). You'll need to boot from either the M.2 flash or a USB flash drive.
Matters arising: In order to fit drives in the space available the plug connecting to the internal speaker has to be removed and about the top 1mm of the plastic latch on the board connector trimmed. If you wish to use the internal speaker you'll need to solder the wires direct to the PCB.
There is still room for the two memory SODIMMs but you will have to remove the metal shield that normally sits over them.
One key thing not to forget is that, by default, the mPCIe socket is disabled in two places in the BIOS. If after all your hard work you find nothing works remember this! You will find the entries in the devices menu and security menu.
Theoretically mounting screws could be used to secure the mounting plate but that is not an easy option. Firstly the disks cover the holes that would be used, and secondly, it's hard to find such tiny screws with big heads/washers.
The STL file is available at thingverse
However he subsequently found that the approach above had a high error rate which he and I put down to the pins in the sockets acting as 'stubs' and screwing up the transmission line characteristics of the cabling. The solution is to either to remove the sockets or to use modified plugs. Pawel took the latter approach as you you can see in the photo. (Not shown is the insulating tape to prevent the connections being shorted out by the metal cover).
Drives: I used a pair of 7mm drives. These end up flush with the casing, but I believe 9.5mm drives should also fit as the upper cover has a ~2.5mm recess inside it.
Here's the caddy in place. You can see the cutout on the left to accommodate the USB plug. At the top left you can see the RAM SODIMM and how close it is to the caddy and hence the need to leave the metal enclosure off.
Here's the completed project. You can see the USB plug/cabling used to power the drives (top right) and the SATA data cabling from the mPCIe SATA controller (bottom left).