The HP t610 thin client and its fatter brother t610-plus are very nice pieces of hardware. They are currently cheap on Ebay and they make very nice network devices such as home servers, IoT servers or OPNsense based firewalls.
If you bought your t610-plus with the hard disk option there is a drive mounting bracket installed. But if you want to add a 2.5" disk after the fact you are out of luck – can you even buy the original mounting bracket?
Adding a standard SATA SSD or HDD is easy enough but holding it firmly in place is a problem w/o the bracket. There are many hacks out there using cable ties, nylon string or cardboard to keep the newly added disk in place. I liked none of them. There already is at least one 3D printable design for a suitable bracket out there but I think it is hard to print without support and so I designed my own version.