A Microsoft file hosting and synchronization service.
And finally, to prove this:
(base) tja@mini:~/Library/CloudStorage/OneDrive-Personal/UNENCRYPTED/MEDIA$ du -ks vlc-record-2014-11-06-22h04m41s-MVI_0582.MOV-.avi EXTRACTED_JPG/2013-03/IMG_20130304_222011.jpg
0 vlc-record-2014-11-06-22h04m41s-MVI_0582.MOV-.avi
1732 EXTRACTED_JPG/2013-03/IMG_20130304_222011.jpg
(base) tja@mini:/Volumes/XTRMQ/OneDrive_TB1/.ODContainer-OneDrive/OneDrive/UNENCRYPTED/MEDIA$ du -ks vlc-record-2014-11-06-22h04m41s-MVI_0582.MOV-.avi EXTRACTED_JPG/2013-03/IMG_20130304_222011.jpg
35728 vlc-record-2014-11-06-22h04m41s-MVI_0582.MOV-.avi
1732 EXTRACTED_JPG/2013-03/IMG_20130304_222011.jpg
Read it from the sync location:
(base) tja@mini:/Volumes/XTRMQ/OneDrive_TB1/.ODContainer-OneDrive/OneDrive/UNENCRYPTED/MEDIA$ cat /Volumes/XTRMQ/OneDrive_TB1/OneDrive/UNENCRYPTED/MEDIA/vlc-record-2014-11-06-22h04m41s-MVI_0582.MOV-.avi > ~/justtoreadthefile
And then check the size in the "cache" (Library) again:
(base) tja@mini:~/Library/CloudStorage/OneDrive-Personal/UNENCRYPTED/MEDIA$ du -ks vlc-record-2014-11-06-22h04m41s-MVI_0582.MOV-.avi EXTRACTED_JPG/2013-03/IMG_20130304_222011.jpg
35728 vlc-record-2014-11-06-22h04m41s-MVI_0582.MOV-.avi
1732 EXTRACTED_JPG/2013-03/IMG_20130304_222011.jpg
Bammmm! The cache on the internal disk got updated (written to) ...
Both reading and writing from / to OneDrives uses a caching system that involves both drives - for all IO!
Of course, this also makes things slower - you are using a second drive as cache.