Why is my external disk being backed up again by Time Machine? 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.
I also use a 256gb external SD card in the MacBook Pro 15” to store music and videos unencrypted. I Have done so for the last 2 years With no issues, while backing up using time machine. @n1000 Interesting that my SD card was never included in the esclude list. So basically I never had to do anything other connect my external hard drive to backup everything on my Mac. I can add the SD to the esclude list manually.