"Disk FOO wasn't ejected because one or more programs may be using it!" (25 Oct 2015)

The weapon of choice in such a case is lsof:

  sudo lsof | grep /Volumes/FOO

In my case, this produced a list of files opened by a process called mds_store which, apparently, is used to produce the Spotlight search index:

mds_store 42059            root  txt       REG                1,6       3277  8832664 
  /Volumes/FOO/.Spotlight-V100/Store-V2/A75D8EF4-8412-4A13-8775-A52C20F05842/live.0.indexGroups
mds_store 42059            root  txt       REG                1,6      32768  8832663 
  /Volumes/FOO/.Spotlight-V100/Store-V2/A75D8EF4-8412-4A13-8775-A52C20F05842/live.0.indexIds
mds_store 42059            root  txt       REG                1,6       8192  8832666 
  /Volumes/FOO/.Spotlight-V100/Store-V2/A75D8EF4-8412-4A13-8775-A52C20F05842/live.0.indexTermIds
mds_store 42059            root  txt       REG                1,6       8192  8832668 
  /Volumes/FOO/.Spotlight-V100/Store-V2/A75D8EF4-8412-4A13-8775-A52C20F05842/live.0.indexPositionTable
mds_store 42059            root  txt       REG                1,6       8224  8832669 
  /Volumes/FOO/.Spotlight-V100/Store-V2/A75D8EF4-8412-4A13-8775-A52C20F05842/live.0.indexDirectory
mds_store 42059            root  txt       REG                1,6       1024  8832670 
  /Volumes/FOO/.Spotlight-V100/Store-V2/A75D8EF4-8412-4A13-8775-A52C20F05842/live.0.indexCompactDirectory
mds_store 42059            root  txt       REG                1,6      65536  8832671 
  /Volumes/FOO/.Spotlight-V100/Store-V2/A75D8EF4-8412-4A13-8775-A52C20F05842/live.0.indexArrays
mds_store 42059            root    5r      DIR                1,6       2074  8832643 
  /Volumes/FOO/.Spotlight-V100/Store-V2/A75D8EF4-8412-4A13-8775-A52C20F05842
mds_store 42059            root   11r      DIR                1,6       2074  8832643 
  /Volumes/FOO/.Spotlight-V100/Store-V2/A75D8EF4-8412-4A13-8775-A52C20F05842
mds_store 42059            root   47u      REG                1,6         28  8832650 
  /Volumes/FOO/.Spotlight-V100/Store-V2/A75D8EF4-8412-4A13-8775-A52C20F05842/indexState
mds_store 42059            root   49u      REG                1,6     118784  8832674 
  /Volumes/FOO/.Spotlight-V100/Store-V2/A75D8EF4-8412-4A13-8775-A52C20F05842/.store.db
backupd   42148            root    4w      REG                1,6       1300  8832803
  /Volumes/FOO/Backups.backupdb/Claus/2015-10-25-175619.inProgress/.Backup.467484979.422497.log

The fun part was that I had explicitly configured Spotlight to ignore that particular external disk - and yet, it was still trying to index it!

Turns out that I am not alone with this. "Disable Spotlight on a FAT32 external drive" provides the best summary I could find. Apparently, the type of file system on the external drive plays a role. I was somewhat skeptical about this claim, but then, all my external drives had FAT32 file systems on them, and so I followed the instructions in the article. I was still somewhat incredulous, as as this seemed to be such a basic issue and the article is rather old.

But then, following the instructions indeed seemed to be successful, at least initially. The magic ingredient in the sauce was to create a top-level file called .metadata_never_index on the affected drive:

touch /Volumes/FOO/.metadata_never_index

I was too impatient to wait for existing instances of mds_store to finish their work, and did not care about the consistency of the Spotlight index on the external drive anyway. So I killed the mds_store process right away, then unplugged the drive and plugged it in again. From there, indeed I had no issues anymore with ejecting the external disk. Hmmm...

Other related articles and discussions:

• http://apple.stackexchange.com/questions/104842/the-volume-cant-be-ejected-because-its-currently-in-use
• http://apple.stackexchange.com/questions/29580/how-do-i-make-spotlight-stop-indexing-my-backup-drive/211616#211616
• http://apple.stackexchange.com/questions/142459/how-to-eject-my-external-drive-when-my-mac-refuses-because-of-spotlight