Linux - find what is preventing you from unmounting a drive

Disclaimer: I've had this problem for probably four months, ever since I started running Plex Media Server on my headless linux machine at home, whilst storing all my actual media on a nice external portable drive. Usually I just yank it, but then I watch the drive letters run themselves up obscenely high before I need to reboot.
So you've gone through the process of mounting your drive in Linux:
> sudo fdisk -l
Device Boot = /dev/sdb1 (and a bunch of other technical information regarding drive size, id, and such)

> sudo mount -t ntfs-3g /dev/sdb1 /media/TOSHIBA

You run a bunch of stuff, get it all working, then find you need to take the drive to work the next day, so you try to unmount it.

 > sudo umount /media/TOSHIBA
umount: /media/TOSHIBA: device is busy.
(In some cases useful info about processes that use the device is found by lsof(8) or fuser(1))

Well, being a Windows guy at heart, and not really understanding why everything here needs to be so technical, this message never told me ANYTHING. I would try typing just plain "lsof" or "lsof(8)" or "fuser(1)" with no useful results.

Then I asked a friend.

"lsof | grep <stuff>," he said.

"Like, <stuff> would be the /dev/sdb1?"


So I tried it:
> sudo lsof | grep /dev/sdb1
mount.ntf     2096      root     3u     BLK    8,33 0x1d1ba997e00       2897182   /dev/sdb1

"Oh," he said. "Try the mount path. /dev/sdb1 is the device which is mounted somewhere else."

> sudo lsof | grep /media/TOSHIBA

"Nothing," I said. "Just a blank line."

"Looks like no program has any file open on it?"

"Let me try again."

bash     2068     daniel    cwd    DIR    8,33     8192    17459  /media/TOSHIBA
grep     2254     root       cwd    DIR    8,33     8192     17459  /media/TOSHIBA
lsof     2255     root       cwd    DIR    8,33     8192     17459  /media/TOSHIBA

"Ahh...It may have been because I was still cd'd into it."


So lessons learned:
  • grep, no matter how weird and complicated it looks with all those pipes and stuff, is still VERY useful. 
  • lsof is the command of choice for figuring this out. 
  • Use the mount point rather than the drive itself when searcing the lsof output text. 
  • Even just navigating into a drive will lock it from being unmounted - it's not like in Windows where if you "Safely Remove Hardware" or yank the USB cable, the computer conveniently closes the explorer window for you.

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