And yet... The Toshiba Canvio external Hdd, formatted with a single partition worked with ALL his 3TB capacity, on that XP computer!
The mistery grows...
Actions that can be taken: 1) Move data somewhere else and re-initialization as MBR (probably the disk is pre-initialized as GPT)
2) Upgrade of the OS to some GPT compatible (it must be a win at least 8, 8.1 or 10? will the other MBR hdd work with one of these? The 8 and 8.1 are generally avoided because of the inavoidable "live tiles", win 10 seems to be rather "curious" for my taste....; w 8.1 handled that older hdd, will w10 work?)
3) Dump completly M$. Use some Linux distro; there 2 problems (at least) arise: a) drivers for peripherals, especially video unit, and b) compatibility (does the linux os access my hdd, or some special program is needed? There is some reserve for using a new, unknown OS....
The M$ article subtly suggest "buy (EVERYTHING: computer, OS, peripherals) new" (or at least that's what I understood from it...)