it is the OS2 Warp 4.52 . Do you think -apart from preparing the volume >2Gb that I'll try to do- Lars'driver installation could be useful in this situation ?
Apparently, Warp 4.52 doesn't have LVM (I thought it did, but your description indicates that it doesn't - there should be no drive letter, until the device is mounted -, or you found a way to install without it), but LVM can probably be installed, if you can find a source for it. I don't remember how to get USB to mount without LVM.
Preparing the device is probably the single most important thing, that must be done. You probably need DFSEE to do it. I think you may be able to do it under windows, or Linux, as long as the device is less than 500 GB, but I never tried that, and I don't know if that would actually help, in this case.
Lars' driver is not likely to help.
In fact, this is all guess work. Perhaps if you posted the CONFIG.SYS, we may be able to spot something. It would confirm whether LVM is, or is not, installed (look for BASEDEV=OS2LVM.DMD), and whether you have the correct statements, in the correct order.
One thing, that might work, would be to mount the USB device (after preparation), in the virtual machine, and then boot. I never tried that either, and I am not sure if it can be done. Another thing to try, is to find a USB stick that is less than 2 GB, format it as FAT16, and see what happens. A 2 GB device, in those days was larger than anything that you could get, without spending a fortune.
The easy answer, is to forget about making USB work, and connect by network, or Shared folders (if that will work). You can also create an ISO file, and attach it as a CD drive, but that is a one way path. USB support is actually a recent addition to VBox, and it seems to need guest OS support to automatically mount the devices.