The HP gt7725 has a standard Phoenix Award BIOS. Press F10 on power up to get into the BIOS screens.
The BIOS is the standard HP one that is common across a lot of their thin clients. The system ROM is identified as 786R6 v1.03. I subsequently updated this to version 2.01 (see below).
It supports three boot devices and, with a USB pen drive in place, I get the options:
I'm not sure exactly what range of USB devices are supported, but I plugged in a USB pen-drive that had Tinycore 8.2 on it and that booted and ran without any problems.
The standard operating system is HP ThinPro - but this had been wiped by the supplier.
It used to be a simple matter to locate and update HP thin client software. Unfortunately I don't find it to be the case these days (October 2017). The HP support pages take me to the relevant product page where, having selected the OS, it always says "No updates available."
Whilst trying to find the original style support pages I came across SP78888.exe which was the "HP Recovery Image & Software Download Tool (ThinUpdate) 64-bit". I downloaded and installed it on my Windows machine. On running it, it promptly updated itself. I then picked the first option: Download a thin client recovery image to local storage or a USB drive.
Next I clicked on Platform and luckily the gt7725 appeared in the list.
The most recent Operating System offered was ThinPro 3.3 1GB x86 Image.
Having inserted a 4GB pen drive I set it going and it formatted the pen drive and then (slowly) downloaded the recovery software to the pen drive.
When I transferred the pen drive to the gt7725 and booted it, it finally ended up bombing out after five attempts to mount the USB key and then an error message:
WARNING: USB Key source and destination device cannot be the same!
I didn't waste any time debugging this. I booted Tiny Core 8.2, inserted the HP prepared pen drive and mounted it. I then unzipped the file images/T6X33009.dd.gz to /tmp. Finally restored the image with dd if=/tmp/T6X33009.dd of=/dev/sda.
For what it's worth I now have ThinPro 3.3 running on the gt7725. (FYI the default administrator password is root).
Googling for the latest BIOS brought up an advisory from June 2011 that said:
System BIOS version 2.03 is available for the HP gt7720 and HP gt7725 thin client models which may improve system thermal performance under certain conditions. Updating the BIOS enables the unit to automatically increase the CPU fan speed as the temperature rises due to an increased work load.
Needless to say the links in the advisory didn't work.
After a fairly fruitless time with Google I finally discovered that BIOS version 2.01 was in SoftPaq SP45871 so I went with that. My first attempt to do the update with the Linux version of the software didn't go anywhere, but on taking the DOS approach it went without a hitch:
Maybe one day I'll find BIOS v2.03. [May 2018: It's in sp52265.exe - thank you Guido]