FAQ
TL;DR: 11 % of PCs still run Windows 7 [StatCounter, 2023]; “most missing-D cases are just wrong partition IDs” says data-recovery expert tzok [Elektroda, tzok, post #16176326] Change the hidden partition’s ID from 12 to 07, assign a drive letter, apply, reboot—data returns. Always back-up first.
Why it matters: A two-minute fix can save hundreds of gigabytes of irreplaceable files.
Quick Facts
• Partition type 0x07 = NTFS/Basic Data; Windows mounts it automatically [Microsoft Docs, 2022]
• Partition type 0x12 = Hidden OEM/Recovery; letters are blocked [Microsoft Docs, 2022]
• Only one active partition per MBR disk; extra active flags break boot [Microsoft KB, 2015]
• Free forensic viewers: DMDE, TestDisk, AOMEI (basic edition) [DMDE, 2023]
• Professional data-recovery costs approx. US$300–1000 per drive [Gillware, 2023]
Why did my D: drive vanish after reinstalling Windows 7?
Setup kept your data but tagged the volume with ID 12 (OEM/Recovery). Windows hides such partitions, so no letter appears [Elektroda, tzok, post #16176326]
How do I make the hidden NTFS partition visible again?
Change its ID to 07, then assign a letter. DiskPart or DMDE can do this safely [Elektroda, NeedArcher, post #16179838]
What’s the quickest command-line fix?
- run diskpart as admin. 2. select disk 0; select part n. 3. set id=07 ; assign letter=D ; exit. Reboot [Microsoft Docs, 2022].
Is changing the ID dangerous for my files?
No data moves; only one byte in the partition table changes. Files stay intact [Elektroda, tzok, post #16180194]
Why are the ‘Change Drive Letter’ options greyed out?
Windows disables the menu when the partition type isn’t Basic Data or when no mountable file-system is detected [Microsoft KB, 2015].
Can I have two active partitions on the same disk?
MBR firmware loads the first active entry it finds. Marking a second one risks a boot loop [Microsoft KB, 2015].
What free tools helped in the thread?
AOMEI Partition Assistant, TestDisk, and DMDE were used to inspect, unhide and retag the volume [Elektroda, NeedArcher, post #16174967]
Could the partition table itself be corrupt?
Yes; if Disk Management shows it as ‘Unallocated’, run DMDE to search lost partitions before writing anything [Elektroda, mati211p, post #16176038]
What’s an edge case where the fix fails?
If the volume was overwritten during reinstall, changing IDs won’t restore data; only sector-level recovery might work [Elektroda, tzok, post #16180194]
How large was the user’s data set?
Disk Management reported 30 GB used and 397 GB free on the 465 GB partition [Elektroda, NeedArcher, post #16179697]
Does Windows 10 handle this differently?
No—both Windows 7 and 10 hide partitions with IDs marked as OEM or Recovery until retagged [Microsoft Docs, 2022].
What’s the annual hard-drive failure rate I should prepare for?
Backblaze logged a 1.4 % annualized failure rate across 235,000 drives in 2023 [Backblaze, 2023].