Time machine backup hard drive and external drive fails in prep stage due to 303 error

I've set up my time machine to include an external SSD in its backup. Initially it worked, then suddenly started failing within 30 seconds during the 'preparing backup' stage. Here is what I believe to be relevant console output: default 10:24:09.412477-0500 backupd Starting query at qos 0x9 default 10:24:09.440069-0500 backupd Starting query at qos 0x9 error 10:24:10.135483-0500 backupd Failed to wait for snapshot deletion to complete on disk '/System/Volumes/Update/mnt1', error: Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted" default 10:24:10.173034-0500 backupd Read (Subarbitration) options: 1 -- URL: -- purposeID: com.apple.TimeMachine.snapshot -- claimID: B5189416-4B1F-41B7-8256-6CC150C2F8E7 default 10:24:10.177676-0500 backupd Subarbitration claim B5189416-4B1F-41B7-8256-6CC150C2F8E7 granted in client default 10:24:10.177701-0500 backupd Subarbitration claim B5189416-4B1F-41B7-8256-6CC150C2F8E7 invoked in client default 10:24:10.189260-0500 backupd Read (Subarbitration) options: 1 -- URL: -- purposeID: com.apple.TimeMachine.snapshot -- claimID: C11E112F-7AD3-4B18-9C70-F800F426C5D3 default 10:24:10.191227-0500 backupd Subarbitration claim C11E112F-7AD3-4B18-9C70-F800F426C5D3 granted in client default 10:24:10.191307-0500 backupd Subarbitration claim C11E112F-7AD3-4B18-9C70-F800F426C5D3 invoked in client error 10:24:10.621504-0500 backupd Failed to mount APFS snapshot with name 'com.apple.TimeMachine.2024-11-12-003546.local' on volume '/System/Volumes/Data' at mountpoint: '/Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/Max's MacBook Pro/2024-11-12-003546/Macintosh HD - Data', error: Error Domain=NSPOSIXErrorDomain Code=2 "No such file or directory" error 10:24:10.622019-0500 backupd Failed to mount reference snapshot: com.apple.TimeMachine.2024-11-12-003546.local source: Macintosh HD - Data error 10:24:11.962968-0500 backupd Unrecoverable zero fileid - path: 'DS_Store', flags: 00011400 error 10:24:11.963092-0500 backupd Unrecoverable zero fileid - path: 'ocumentRevisions-V100/.cs/ChunkStoreDatabase', flags: 00010400 error 10:24:11.963161-0500 backupd Unrecoverable zero fileid - path: 'umentRevisions-V100/LibraryStatus', flags: 00018800 error 10:24:11.963244-0500 backupd Unrecoverable zero fileid - path: 'umentRevisions-V100/LibraryStatus', flags: 00015800 error 10:24:11.963310-0500 backupd Unrecoverable zero fileid - path: 'umentRevisions-V100/db-V1', flags: 00024000 error 10:24:11.963365-0500 backupd Unrecoverable zero fileid - path: 'umentRevisions-V100/purgatory', flags: 00024000 error 10:24:11.963417-0500 backupd Unrecoverable zero fileid - path: 'umentRevisions-V100/staging', flags: 00024000 error 10:24:11.963468-0500 backupd Unrecoverable zero fileid - path: 'poraryItems/folders.0/TemporaryItems/NSIRD_revisiond_IE36Cr', flags: 00020300 error 10:24:11.963572-0500 backupd Unrecoverable zero fileid - path: 'poraryItems/folders.0/TemporaryItems/NSIRD_revisiond_IE36Cr/LibraryStatus', flags: 00010900 error 10:24:11.963774-0500 backupd Event collection stopped because of error: Error Domain=com.apple.backupd.ErrorDomain Code=303 "(null)" error 10:24:11.964286-0500 backupd Failed to collect events for "max_music" (device: /dev/disk4s1 mount: '/Volumes/max_music' fsUUID: 03273473-8B2C-3D62-BB73-6640675B8651 eventDBUUID: 8E7383AE-643C-48A5-8DE7-86E23DCBE65A), error Error Domain=com.apple.backupd.ErrorDomain Code=303 "(null)" UserInfo={FailedStrategy=16} error 10:24:12.281645-0500 backupd Backup failed (303: BACKUP_FAILED_PREFLIGHT_STAGE_GATHER_EVENTS) Hard to know what I could be doing to trigger it. My SSD is used for Logic Pro sessions and other files and I regularly work from the sessions that live there. Sometimes I move large amounts of files to that drive to clean out my hard drive. The first time this issue started it lasted 3 weeks before resolving itself (no idea why). In summary - please help me: - further investigate the issue - resolve this issue - setup an alternate way to automatically backup my external SSD (maybe the way I'm doing it is not recommended?) Things I've tried: - first aid-ing the time machine drive and the SSD: no issues - manually deleting in progress and interrupted backup files - toggling the spotlight index privacy to exclude my SSD - waiting awhile while drives are connected (only thing that has worked in the past) - restarting while drives connected - talking to my computer