Okay I've not a T540p which operates ArcaOS and W10 in parallel. I didn't get booting from ArcaOS install stick to work. Think I've tried booting from DVD too without success. But not sure anymore.
Anyway using a preinstalled eCS / OS/2 / ArcaOS works. The 'OS/2 is unable to operate your hard disk' most probably was caused by problematic partitioning of the inserted drive. Although I use dfsee running under OS/2 for almost all partitioning task it seems I've set up something wrong. Anyway after starting from scratch it works. I usually create at least on primary partition. Most of the times I create the IBM boot manager partition. Although for booting I use AirBoot afterwards. My usual disk layout includes at least two or three OS boot partitions, a programs partition, a data partition and a temp partition. Preparing the disk on another system and installing it into the T540p works. It even runs the display at 2880x1620 resolution :-).
Preinstalled W10 - I created a recovery stick for that before changing GPT to MBR. I did not find a way to preserve the partitions when converting to MBR so I deleted all and hoped the recovery stick will do his job afterwards. It didn't. I created the Windoze partitions with dfsee beside my OS/2 partitions. The recovery stick let me select the proper partition and starts to install and runs a long time until about 85%. But then it gave up. Tried a few times and all the Win startup problem determination tools without success. I can only guess the W10 won't recover cause I prohibited all internet access ;-) I gave up on installing W10 on the same drive as my OS/2 partitions and ordered a 120GB M2 SATA dedicated to W10. Now I've the full 250GB SSD for OS/2. W10 runs from the second hard disk in the M2 slot.
I downloaded a fresh W10 version from Microschrott website. Someone told me I should do this from an older non UEFI/GPT system. MS seems to prepare different W10 versions for different customers. From my OS/2 system I got a link for downloading 32 and 64 bit versions. I prepared partitions on the M2 SATA with dfsee on another system. Then I installed W10 to the T540p in the prepared partition. For safety reasons I get out the SSD with OS/2. After playing around a while and set up some important things (real shutdown, ...) I now wonder what I should do with this unloved toy. Putting back the SSD and now W10 works beside OS/2. Boot selection is done by AirBoot. That's enough for now.
Now I've to adapt my OS/2 for this system. Fonts and symbols are that small on this display. It's the first time I think I need bigger icons. LAN seems to work. USB mouse on docking station works. Let's see what else. Btw. the Win setting to increase all fonts to 150% isn't that bad.
Additional hints - I upgraded the BIOS to 2.37. Machine Type Model is 20BE0086GE