It is very unlikely that the Saving tag changes "item" caused the buffering issue. The problem is elsewhere, and they're both symptoms of the same problem.
The message you're seeing occurs when MC is trying to write a tag out to a file (which could be updates to play stats, for example, or because you manually tagged something). If MC can't write to the file, then it waits and tries until it "gets through". If that message is persisting down at the bottom, it means the file has continued to be write locked.
This is normal while a track is playing. MC can't write to it while it is being played, but as soon as you "stop" or it switches to the next track, it should quickly finish the remaining task and that message should go away. The write is not very data intensive, and done at a low priority, so it should not impact real-time playback in any way (and even if it did, the write isn't actually happening currently if you're seeing that message, because it couldn't write). Until the write actually happens, the throughput for reading on the disk would be unaffected (aside from maybe one quick seek occasionally).
So...
You're seeing difficulty writing to files on disk, suddenly. And, concurrent with this...
Difficulty playing back files.
Sounds like disk access problems to me. Disk formatting issues (run chkdsk on the volume)? Disk permissions issues (check them in Windows)? Or... Disk hardware problems? It might be failing.
I'd do some testing on that source disk.