Windows doesn't show the drive at all

Which of these is happening

Open Disk Management (right-click Start, then "Disk Management") and compare. The distinction matters, because four of these are data problems and the last one is a hardware problem.

The whole disk shows as "Unallocated"

The disk is present and its size is right, but Windows sees no partitions on it. The partition table has been damaged, overwritten, or replaced. The volumes themselves are still on the disk at the addresses they always occupied — nothing has moved, and nothing was erased when the table was lost.

"You must initialize a disk before Logical Disk Manager can access it"

The same situation with a dialog attached. Initialising writes a fresh, empty partition table. Click Cancel. It is not a repair and it does not make the data easier to find; it replaces the record of where your volumes are.

A partition is listed, but there is no drive letter

Windows can see a volume and has decided not to mount it — commonly because it is a file system Windows does not read, because it is encrypted, or because the letter assignment was lost. If Disk Management describes it as "RAW", read the drive asks to be formatted, or shows up as RAW instead; that page covers the same volume from the other side.

The disk came out of a NAS, a Linux machine, or an Android device

Then Windows is behaving exactly as designed and the disk is fine. It holds ext4, or XFS, and very often those live inside an LVM container rather than in an ordinary partition — a layout Windows has no concept of at all. Every consumer recovery tool that works through the Windows volume API reports an empty disk here, which is indistinguishable, to the person holding it, from a disk that died.

The disk is not in Disk Management at all

This is a hardware or connection problem rather than a file system one: cable, enclosure, power, or the drive itself. Try another cable and another port, and try the bare drive in a different enclosure or connected directly. If the drive makes noises it did not use to make, stop — see a failing drive: copy it before you scan it.

What not to click

• Initialize Disk. Writes a new, empty MBR or GPT over the old one.
• New Simple Volume. Creates a partition and formats it, on top of your data.
• "Assign drive letter" followed by an offer to format. The offer to format is the same trap as everywhere else on this site.
• Partition repair utilities that write. Some genuinely can rebuild a table, and they are the right tool eventually — but every one of them writes to the disk, and a wrong guess written to the disk is much harder to recover from than the current situation. Copy the files off first.

How a lost volume is found again

Recovery software that reads the medium itself, rather than asking Windows for a list of volumes, has three ways to work out where a volume starts. Unlost tries them in order of how much each has to assume.

1. The MBR partition table — four entries at the end of the first sector, plus the chain of extended records that logical partitions hang from.
2. The GPT — a header, a table of entries, and a CRC over each, plus a complete second copy at the far end of the disk. That backup copy is what makes a destroyed partition table survivable: the table can be read from the end even when the front of the disk is unusable.
3. A sweep of the medium itself. Every sector is tested for the start of a file system, and a hit is only believed once the parameters behind it have been parsed and made to agree with each other. This is the slow one, and it is the one that finds a partition nothing on the disk records any more.

Two details decide whether this works on a modern disk. Addresses in a partition table are counted in the medium's own sectors, and a 4K-native disk states everything in four-kilobyte units — a table read in 512-byte units puts every partition at an eighth of its real address, which is not slow but wrong. And an LVM label found where a file system was expected is followed into the volume group, so the logical volumes inside come back as ordinary entries with real addresses.

File systems with no reader on Windows — APFS, HFS+, btrfs, ReFS, ZFS, UDF — are named rather than passed over. Knowing that a disk holds an APFS volume is a fact you can act on; being told the disk is empty is how a healthy disk gets thrown away.

What to do

1. Cancel every dialog offering to initialise, format or create a volume.
2. Check the physical connection first if the disk is not listed at all. Different cable, different port, different enclosure. A disk that is not detected cannot be read by anything.
3. Install recovery software on a working drive and let it look at the whole disk rather than at a drive letter, since there is no letter to point it at.
4. Let it search the medium for volumes. On a large disk this takes a while — it is reading every sector — and it is the pass that finds partitions no table records.
5. Recover to a different disk with room for everything, and open a sample of the files before you touch the source disk again.
6. Repair or repartition afterwards, if you want the disk back. Once the files are safe, writing a new partition table is a normal thing to do.

Questions about missing drives

Should I let Windows initialise the disk?

Not while you still want the data. Initialising writes a new, empty partition table. It does not erase your files directly, but it removes the record of where they are and makes the disk look ready for use — after which anything Windows writes can land on them.

The disk is listed in the BIOS but not in Windows. What does that tell me?

That the drive is alive and connected, and the problem is on the partition or file system side rather than in the hardware. That is the recoverable kind of problem.

Does it matter whether the disk was MBR or GPT?

For you, no — recovery reads both, and a GPT disk has the advantage of a complete backup table at the end. It matters only if you later rebuild the partition table by hand, where the two are written differently.

My external drive from a Mac shows as unallocated. Is it recoverable?

The disk is readable, and its layout can be identified — an APFS or HFS+ volume will be named as such rather than skipped. Reading the files inside an APFS volume is a separate matter, and Unlost does not do it: it will tell you what is on the disk, which is what lets you take it to the right tool or back to a Mac.

Can a disk lose its partition table on its own?

Rarely on its own; usually something wrote to the first sector. A failed format, an installer pointed at the wrong disk, an enclosure that lost power mid-write, or a tool that "prepared" the drive. The good news in all of those is that only the first sectors were touched.