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Partitioning of 4TB HDD for legacy OSes (Users)

posted by RayeR Homepage, CZ, 28.04.2026, 17:21
(edited by RayeR on 28.04.2026, 17:33)

> In theory, int 13, ah=48h, dl=drive, ds:si pointer to buffer is supposed to
> return a 64-bit value at offset si+10h. I can easily imagine a BIOS failing
> to return all bits here, though. Which tool did you use to view the EDD
> data?
Many years ago I wrote a small utility EDDINFO. I looked in my source and found that I ignored the high Dword and printed only the low DWord because of old Borland C don't support printing format %llu (is there some specific format string for BC or any?). So I quickly modified the code to just print both DWords separately but I still get 0 in high Dword so yes, the BIOS/EDD is really buggy/too old...

I already tried to replace disk.sys, partmgr.sys and mountmgr.sys from Windows Server 2003 SP2 as suggested on many forums but nothing has changed! After some digging info through AI it told me, that in my specific case - SATA controller in IDE compat. mode when it use standard IDE driver (instead of e.g. AHCI driver or RAID driver), then the LBA is passed to the NT kernel via NTDETECT.COM / NTLDR from EDD and it has HIGHER priority than what driver might detect via ATA identify. I got a suggestion that I can try patch EDD runtime via GRUB4DOS and then chainload NTLDR - grub command geometry (hd1) should detect the correct LBA, then if drivers from W2K3 will work as expected I should see full capacity...

I also tried to look at the HDD from Win7-x64 and the same tools like 32-bit HDTune reported 4TB correctly, so this tools rely on what they get from the OS. Using Win7-x64 Disk manager I was able to create one smaller partition ~370GB that filled the rest from 1,6 to 2TB limit. It sees next 2TB of unallocated space but don't allow to create partition there while still having MBR-only. Maybe if I filled the values in MBR manually... But first I'd like to solve EDD...

> As people already started to work on this, I suggest to contribute to DOS
> GPT support if you really want > 2 TB for DOS.
I don't need so much to access that 2nd partition over 2TB, it would be probably NTFS anyway. But just need that DOS wouldn't complain and I can access all below 2TB.

> Does hybrid GPT-MBR really work? Or is the MBR on a GPT disk just a dummy
> to keep non-GPT OS from causing problems?
There are 2 kind of MBR when GPT involded. Normally all tools generate protective MBR that is just dummy, useless for DOS. But Linux gdisk can (as I belive) generate valid MBR from GPT partitions and vice versa automatically so you could have up to 4 partitions defined same as in MBR and GPT so both legacy and modern OSes should access it....

---
DOS gives me freedom to unlimited HW access.

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