My iMac had an SSD startup drive (called SSD) and a 3TB internal hard drive (MacintoshHD) duplicating the SSD as well as containing the User folders and all data.

MacintoshHD died last week. I reinstalled the original 1TB hard drive (also called MacintoshHD but not currently being used) and bought an external Thunderbolt 4TB drive. Onto this I've copied my home folder (at root level so the supplied LaCie setup software is untouched in the root too).

I have successfully restored everything but my Movies and Downloads folders (which stupidly I excluded from TM because of space issues!) by copying the contents of the .../Backups.backupdb/Macintosh/Users/operacentric folder to the new drive. Thus the new drive doesn't duplicate all users and system folders that the original had.

I want Time Machine to continue using the existing backup set (at least until I can afford an 8TB new drive when I can start again!) It has backups for SSD and MacintoshHD until the last backup but one (which is when the HDD failed). So the Latest folder contains an SSD backup but the previous one has folders for both drives.

Having read the manual page and various guides here and elsewhere about TMUTIL, I believe I can use Associatedisk to relink the backup to the new drive. However, they don't deal with the issue of having two drives in the backup. I am not quite sure of the syntax - do I use the root drive name (LaCie), the Users folder or the operacentric folder?

sudo tmutil  associatedisk [-a] mount_point snapshot_volume

Is the mount_point:

  • /volumes/LaCie or
  • /Volumes/LaCie/Users or
  • /Volumes/LaCie/Users/operacentric ?

Can I use the actual snapshot volume rather than the (missing) link in Latest?, so


or do I enter the same point of the hierarchy


All the posts I've seen here relate to a single drive - not sure how to work with the internal and the external...

  • @grgarside - sorry for rambling! Is this clearer? List of disks: 1. SSD (480GB - Internal) - Startup Disk (System and Users folder for Admin and Guest User accounts) 2. LaCie (4TB - external) - Data Disk (User folder for Operacentric account) which replaced the dead 3. MacintoshHD (3TB - internal) - Data Disk (all User folders plus duplicated System) 4. TimeMachine (4TB - external) - Backups disk We can ignore 5. IntMacintoshHD (1TB - internal) - original Mac disk reinstalled but not in use 6. MacintoshHD (2TB - external) - used for temporary restore of User data
    – goldnet
    Jan 5, 2018 at 18:05
  • Time Machine Backups: Screenshot of last completed backup - link Screenshot of ‘Latest’ folder - link Screenshot of LaCie - link
    – goldnet
    Jan 5, 2018 at 18:06
  • So my confusion is mainly because: a) I am not using the contents of the "Latest" TM folder as per instructions (because it is incomplete and doesn't contain a backup of the HDD) and b) the folder hierarchy of the old and new data drives is different
    – goldnet
    Jan 5, 2018 at 18:09
  • You asked about partitions - only single partition on each here is Disk Utility's summary - SSD link. LaCie HDD - Link. TimeMachine - Link
    – goldnet
    Jan 5, 2018 at 18:13
  • Latest: I used associatedrive and thought I'd done OK - TM started, used existing drive, didn't complain it's full, appeared to continue existing backup, renamed MacintoshHD to LaCie, got as far as cleaning up backups (so checked folders were all correct and intact and went to bed). This morning all backups except the last (which only had startup drive data in it) are wiped. TM started a new BU from scratch, which will take 3-4 days: exactly what I didn't want to happen! I wanted to preserve a full set of snapshots before switching to new 8TB drive specifically for a new set.
    – goldnet
    Jan 6, 2018 at 16:42

1 Answer 1


I am extremely confused as to what names are what disks and partitions and such in your particular scenario. Therefore I will attempt to provide some guidance on the ‘associatedisk’ verb and how to use it, but I can't say for sure the exact commands to run with the names of your existing disks as I don't know what's what.

The associatedisk verb takes a mount point and a snapshot volume, as you found from the man page. This means if you are backing up a disk named "Macintosh HD" on a computer named "George's MacBook" to a Time Machine backup disk named "Time Machine HD", to reassociate this disk to the backup you would run:

sudo tmutil associatedisk -a "/Volumes/Macintosh HD" \
  "/Volumes/Time Machine HD/Backups.backupdb/George's MacBook/Latest/Macintosh HD"

The -a flag associates all other snapshots of the same volume to the disk. This might be useful as you mention something about ‘Latest’ link being broken — point at a specific date/time and use the flag so that the other snapshot dates are associated too.

Feel free to add to your question with a list of disks, partition names and what their purpose is in your setup and I might be able to provide some specific commands to run.

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