I’ve a headless Mac mini that has a SMB-shared quantity that my MacBook Professional is configured to backup to through Time Machine. Each are working the newest model of macOS Sequoia (15.3.2).

I’m attempting to alter the password of my encrypted community Time Machine backups and am at present hitting a wall. To my information this isn’t a documented characteristic for community Time Machine backups, however since Time Machine makes use of sparsebundles and there is a utility to alter their encryption keys I assumed Time Machine may help it.

What I’ve tried to do:

1. Change encryption password of sparsebundle by utilizing sudo hdiutil chpass /Volumes/Backups/<identify>’s MacBook Professional.sparsebundle
   • I’ve Confirmed that I can manually mount <identify>’s MacBook Professional.sparsebundle (by double clicking it and getting into the brand new password)
2. Manually kick off Time Machine backup, it fails with a message that I must replace my password. So I click on “Replace password.”
3. It asks me to enter my new password – I enter the brand new password and click on Enter
4. After a number of seconds, I see the identical error display screen – the brand new password is being reported as incorrect (regardless that as talked about I can open the sparsebundle myself with the brand new password)

I’ve checked out syslogs for backupd and located a number of that look attention-grabbing:

2025-03-18 22:54:32.936183+1100 0x9937a    Default     0x77ba5              514    7    backupd: (DiskImages2) [com.apple.DiskImages2:Default] +[DIEncryptionFrontend hasGUIaccess]: Person session flags: 0x1 (0x1=root, 0x10=graphics, 0x20=hasTTY, 0x1000=IsRemote), secRes=0
2025-03-18 22:54:35.039877+1100 0x9937a    Error       0x77ba5              514    7    backupd: (DiskImages2) [com.apple.DiskImages2:Default] +[DIError errorWithDomain:code:description:verboseInfo:error:]: Can't retrieve passphrase from person through TTY or GUI (err code 25)
2025-03-18 22:54:35.039967+1100 0x9937a    Default     0x77ba5              514    0    backupd: (libxpc.dylib) [com.apple.xpc:connection] [0x625bd8a00] invalidated as a result of the present course of cancelled the connection by calling xpc_connection_cancel()
2025-03-18 22:54:35.040084+1100 0x9937a    Error       0x77ba5              514    0    backupd: (TimeMachine) [com.apple.TimeMachine:DiskImages] Failed to connect utilizing DiskImages2 to url '/Volumes/.timemachine/<host>/ADD2A9B1-B032-4287-B688-2277179038A2/Backups/<identify>'s MacBook Professional.sparsebundle', error: Error Area=NSPOSIXErrorDomain Code=25 "Inappropriate ioctl for gadget" UserInfo={DIErrorVerboseInfo=Can't retrieve passphrase from person through TTY or GUI}
2025-03-18 22:54:36.307690+1100 0x99379    Error       0x77ba5              514    0    backupd: (TimeMachine) [com.apple.TimeMachine:BackupDispatching] Backup failed: BACKUP_FAILED_DISK_IMAGE_WRONG_PASSWORD (53)

From the above it seems that DiskImage2 is unable to get the brand new password ("Can't retrieve passphrase from person through TTY or GUI (err code 25)"). I’m undecided why that is taking place, as I’ve offered the password when it prompted me to replace it. Perhaps this isn’t being handed on to the DiskImage2 course of that is working as root?
I’m undecided, nevertheless it undoubtedly looks like it ought to work given it recognises the password wanted to be up to date. I’ve additionally tried eradicating the outdated password from my login and System keychains in Keychain Entry in any file I may discover that regarded related, nonetheless the identical error.

Solely repair I’ve discovered is to alter the password again to the unique (utilizing the identical sudo hdiutil chpass command). After altering the password again, I can begin a handbook backup and it may possibly mount the sparsebundle efficiently. I am questioning if which means that Time Machine shops a replica of the sparsebundle password elsewhere?

I’ve additionally learn that Time Machine does some truncating of the password (https://apple.stackexchange.com/a/331768/621735). The unique password is saved in my 1Password vault and I can see that Keychain Entry has a truncated model of it saved (26 characters). I’ve tried passwords <26 characters and I am nonetheless working into the identical situation, so I do not suppose it is associated to this however I could possibly be mistaken?