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?
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.
There seems to be a certain bug in macOS 10.14.4 where encrypted drives fail to auto-mount. I have a feeling this could be related to my issue.