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My 200 GB music library resides on a SD card that is permanently inserted into my MacBook Pro. The volume is encrypted using Filevault. Time Machine will include that volume when doing backups onto an external USB disk. This solutions worked very well for the past 1–2 years.

However, today Time Machine decided to backup the entire SD card again (I verified this using BackupLoupe) and it is a complete mystery to me why it did that. The md5 checksums of both versions are identical and also permissions seem identical between the two backed up versions and the original one. My main drive was backed up as usual (i.e. no full backup).

The only actual change I can think of, is an upgrade to the macOS 10.14.4 from 10.14.3.

Any ideas why these files have been backed up again and how I can investigate this? Is there a tool / command to see why TimeMachine decides to back up a file?

Update 1

It just happened a second time and again the entire disk is being backed up. So I am starting to suspect that something is happening which leads Time Machine to believe this is a new disk. As I said, this worked without issues in the past.

And, yes, I am aware that this is not the safest (but cheapest ;) form of data storage and I keep redundant backups.

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  • I hope you have a backup of your music library. These cards are not as reliable as ssd's. – historystamp Apr 3 at 3:27
  • I do the same thing, but how did you get Time Machine to backup that disk? I only know how to exclude things from a backup, not how to add other disks. – dwightk Apr 4 at 14:29
  • @dwightk If I remember correctly, external disks are automatically added to the list of exclusions and I had to remove it from there, i.e. System Preferences > Time Machine > Options... Also, I think only APFS / HFS+ file systems can be backed up. – n1000 Apr 4 at 14:33
  • huh... yeah it's showing any external disk that I add there except my sd card... All of them are Mac OS Extended (Case-sensitive, Journaled) which I think is HFS+ (anyway, comments are really the spot for asking questions like this :D) – dwightk Apr 4 at 14:48
  • @dwightk Yes, why not start a new question? – n1000 Apr 16 at 9:05

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