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Partitioning of 4TB HDD for legacy OSes (Users)

posted by RayeR Homepage, CZ, 27.04.2026, 22:45
(edited by RayeR on 27.04.2026, 22:56)

I just upgraded my old 1TB HDD to 4TB (beside a SSD for system, attached to intel P67 onboard SATA 3.0 controller in IDE mode). I like to have most of the capacity accessible for my legacy OSes (DOS, Win9x, NT, XP...). On old 1TB I had nonLBA ext. partition with logical drivers that is accessible in MS-DOS 6.22 and the rest was big FAT32. Now I hit the 2TB barrier of MBR. But I wonder that I hit even lower. Neither FD-FDISK nor WinXP disk manager is able to create partition larger than 1,6TB. WTF? Why not the whole 2TB? Then I found that is seems my BIOS is too stupid to report limited EDD capacity that then fools the FDISK and even the Windows (I would expect WinNT/XP use their own disk drivers and don't rely on EDD values. Here is it:

EDD version: 2.1
EDD features: Fixed disk access, EDD support
EDD flags: 0000h
Physical drive #1 geometry LBA sectors: 3519069872 (1803 GB), sector=512 B
Device Path Information: absent
Device Parameter Table Extension pointer: F000:8420h
DPTE revision: 1.1 [OK]
DPTE flags: 2897h
Controller I/O base address: 1F0h, control port: 3F6h
IRQ: 14, DMA channel: 2
PIO mode: 4, DMA mode: 0, Ultra DMA: enabled
32-bit transfer mode: enabled, LBA: enabled, LBA48: enabled
ATA read/write multiple command max sectors: 16, enabled
ATAPI: no, uses command packet interrupt: no, nonremovable media
Host Protected Area: inactive

As you can see LBA sectors: 3519069872 (1803 GB) instead of real drive capacity that is exactly 7814037168 sectors. So it's an evident EDD bug - 7814037168 mod 2^32 = 3519069872 so BIOS do only 32b arithmetics...
Maybe it would be easier to fix some table in memory after boot than BIOS patching but less universal. For WinXP there seems it may be a solution to replace some *.sys drivers from Win 2k3 SP2 ther should correctly handle LBA48. Linux (even old 32b Debian) seems to habdle it correctly.

Then need to deal with 2TB MBR barrier. I'm thinking to create a hybrid MBR with GPT. The GPT would mirror the existing MBR-defined partitions and adds one GPT-only partition behind 2TB barrier that will be accessible only for newer OSes (probably NTFS). I think it should be safe when MBR and GPT will keeps synced. Linux gdisk should have options to create this. Win98 should have working LBA48 fix for ESDI_506.PDR, WinXP should use GPT via Win2k3 drivers but not sure about NT4. Any idas?

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

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