Time Machine drive not mounting: what to try, and what Retrova can read

A Time Machine disk that won’t mount has usually lost a clean file system, not your backups. Check the connection and Disk Utility first, and don’t erase it. Retrova can read both APFS and older Mac OS Extended (HFS+) backup disks read-only, including encrypted APFS ones if you have the password.

By Jishnu Mahanta · Updated 2026-10-20

In short

Don’t erase the disk or let Time Machine “start a new backup” on it. Check it in Disk Utility. Retrova can read the volume read-only whether it is APFS or HFS+. On APFS it doesn’t browse Time Machine snapshots; on HFS+ the backup folders open like normal folders. If the backup is encrypted, have the password ready.

First, work out what kind of Time Machine disk you have

Since macOS Big Sur (11), Time Machine can back up to APFS disks, and APFS is the default for new backup disks. Older backup disks, and any disk set up before Big Sur or deliberately as Mac OS Extended, use HFS+ (Macworld, Eclectic Light). Time Machine can also encrypt a backup disk if you ticked that option. You can see the format with:

Open Disk Utility and choose View > Show All Devices. Select the volume: the right-hand panel shows “APFS” or “Mac OS Extended (Journaled)”.
Or in Terminal, diskutil list and diskutil info disk4s2, both read-only.

Gentle checks first

Cable, port, power. Plug straight into the Mac. Spinning backup drives often need their own power. Try another cable.
Give it time. Large Time Machine disks can take a long time to mount after an unclean eject because macOS checks them. Wait before repeating anything.
Is it encrypted? If macOS asks for a password when the disk is connected, enter it. If you’ve lost it, the backup can’t be unlocked by anyone, including Retrova. If you have it, Retrova can ask for it too (see below).
Try Mount in Disk Utility. If it fails with an error, see “Could not mount”: com.apple.DiskManagement.disenter error.
Time Machine’s own browser. If the disk does mount, Option-click the Time Machine menu bar icon and choose Browse Other Backup Disks…, then copy what you need.
First Aid writes to the backup disk. A backup is often the only copy of old versions, so copy first if you can. If it fails, see Disk Utility First Aid failed (exit code 8).

If it is an APFS backup disk: Retrova

Retrova reads APFS read-only and can open a volume macOS refuses to mount. Two honest limits: it doesn’t browse APFS snapshots, and Time Machine on APFS keeps each backup as a snapshot. So what you see in Retrova is the volume’s current contents. How much of your backup history that includes depends on how Time Machine laid out the volume, and I can’t promise it will show every dated version. Try it on a copy of what you need and check the files open.

Plug in the disk and open Retrova. Choose A drive won’t open.
Pick the APFS volume. Retrova asks for your administrator password to read the disk.
Browse and select the folders or files you want. Preview a few before copying.
Copy to a different drive, then Verify.

If the backup disk is encrypted, Retrova marks it “Locked”. Enter the password and the files open as usual. Decryption runs at about 100 MB/s, so a large backup disk is slow to read. If you would rather not enter it in Retrova, unlock the volume in macOS (diskutil apfs unlockVolume) and copy from Finder instead. See Recover files from an APFS drive that macOS won’t mount for APFS in more depth.

If it is an HFS+ (Mac OS Extended) backup disk

Retrova can read Mac OS Extended (HFS+ and HFSX) disks, including older Time Machine backup disks. It reads the hard-linked folders Time Machine uses, so each backup folder browses like a normal folder and you can open one and copy out what you need. Names with accents and emoji come out right, and compressed files and resource forks are read.

Plug in the disk and open Retrova. Choose A drive won’t open.
Pick the Mac OS Extended volume and enter your administrator password so Retrova can read the disk.
Open the backup folder you want, select what you need and preview a few files.
Copy to a different drive, then Verify.

One limit: Retrova doesn’t replay the disk’s journal. If the disk was unplugged without ejecting, changes made just before that may be missing, and Retrova says so when it sees it. If the disk is damaged beyond reading, Deep scan can still find photos, videos and documents by content, but without names or folders. macOS itself (Time Machine’s Browse Other Backup Disks, Migration Assistant) and the free TestDisk and PhotoRec are other options. See TestDisk vs PhotoRec vs Retrova.

If the disk is failing

Backup disks run for years and fail. If it is slow to respond, disconnects, or makes noises, image it before browsing (Image a failing drive on Mac or Windows). If it clicks or grinds, stop and talk to a recovery lab.

What Retrova can’t do: it doesn’t browse APFS snapshots, can’t open an encrypted drive without its password or recovery key, can’t open the startup disk of a T2 or Apple silicon Mac away from that Mac, doesn’t handle RAID sets or phones, and can’t fix a drive with physical damage. Retrova’s builds aren’t signed or notarised yet, so macOS and Windows will show a warning the first time you open it.

What not to do

What not to do

Don’t let Time Machine “erase and start a new backup” on the disk. That erases the history.
Don’t click Initialize or Erase in macOS dialogs.
Don’t run First Aid again and again on a drive that is slowly failing.
Don’t assume the backup is empty because the disk won’t mount. It usually isn’t.

Get Retrova

Free and open source for macOS 11+ and Windows 10/11. No account, no recovery limit, and it never writes to the drive you’re rescuing.

Download RetrovaFree for Mac and Windows

Questions

Can Retrova read my Time Machine backup?

Yes, in most cases. Retrova reads APFS and Mac OS Extended (HFS+) backup disks read-only. On an HFS+ Time Machine disk the backup folders browse like normal folders. On APFS it doesn’t browse snapshots, so don’t expect every dated version. An encrypted APFS backup disk opens if you have its password.

How do I know if my Time Machine disk is APFS or HFS+?

Select the volume in Disk Utility with Show All Devices on: the panel shows APFS or Mac OS Extended (Journaled). Disks set up under Big Sur or later are normally APFS.

Will I lose my backups if the disk won’t mount?

Not necessarily. Don’t erase it or start a new backup on it. Often the disk can be recovered, and the backups with it.

My Time Machine backup is encrypted. What now?

You need the password. Retrova opens an encrypted APFS volume when you enter it: the drive shows as Locked, you type the password, and the files open like any other drive. The password stays in memory only. Without it, nobody can open the backup. An older encrypted Mac OS Extended backup disk may not open in Retrova; if so, unlock it in macOS and copy from Finder.