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High Sierra: won't mount external volume not mounted at start-up

When running High Sierra, can I manually mount an external drive volume that was explicitly prevented from mounting at start-up? (I can't do it with Disk Utility).


I've known for a long time how to configure etc/fstab to prevent MacOS from mounting external drive partitions. But in the past (until I upgraded to High Sierra ... yeah ... and belatedly learned about the switch from HFS+ to APFS) I was always able to use Disk Utility to mount partitions that had been explicitly prevented from mounting at star-up.


I had been using a single external (USB) drive containing 3 partitions when I first upgraded to 10.13.6, and I could see these "partition's" on the external drive when using Disk Utility, but only the partition I had been using for Time Machine backups was mounted. The other two were greyed out. Attempts to mount either of the other two partitions were ignored - no action and no feedback. (These partitions also had been set up for use by Time Machine).


Remembering that my etc/fstab was configured to prevent mounting these two volumes at start-up, I cleared etc/fstab and re-booted. Bingo! All three external volumes mount at startup and all 3 can be mounted/dismounted/remounted manually via Disk Utility.


So yeah, I can mount those volumes, but if I only want to use them under rare circumstances, it's kind of a hassle to have to muck with fstab.


I'm guessing this problem has something to do with telling the file system to use smb to access the rarely used volumes, but I cannot figure out how to do that.


Can someone help me out?

Posted on Aug 30, 2019 1:55 PM

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14 replies

Aug 30, 2019 7:28 PM in response to BDAqua

Sorry to be dense but, if 'File Sharing & Favorites' is a choice within a particular app (Finder for example), I don't know what you are talking about.


I know there is a 'File Sharing' option in the pane that pops up when you select System Preferences/Sharing. However, that File Sharing value has historically been disabled (unless I'm allowing another home mac on our LAN to have access). However, I did enable it temporarily, and selected SMB only, then tried to mount a greyed out volume in Disk Utility ... to no avail. And by the way, when the additional partitions are not disabled at start-up, they pop up on the desk top (mounted) even though File Sharing is disabled (Time Machine doesn't have a problem making backups to the external partition (USB external drive) that's the historical home for the machine I'm using) .... so I don't know why the 'File Sharing' option (which I thought was for network connected file transfers) would have an effect on this issue.


A lot of administrative apps have a Favorites option, so you'll have to be more explicit about the app that offers the Favorites option that lets me configure a file transfer protocol. (If indeed it is a separately configured item from File Sharing)

Aug 31, 2019 2:56 PM in response to BDAqua

Tried that at your suggestion but:


External USB drive volumes, whether mounted or unmounted do not appear in the list of favorites or recently connected servers. I have been unable to determine the proper access handle (smb://???) for connecting to the USB drive. I can see that the handle for connecting to other machines on my LAN is, for example, smb://othermac%201._smb._tcp.local, for a computer named othermac1, but I don't know how to connect via a server that would use smb protocol to talk to a USB drive ... especially if a partition on the drive is not a mounted volume.

Sep 2, 2019 6:12 PM in response to BDAqua

Yeah this is NOT about shares on the network. It's about volumes (HFS+ partitions) on an an external drive connected to a mac. As I said in my original question. These volumes can be mounted at startup if not disabled in etc/fstab. However, if they are disabled at start-up, I can find no way to mount them even though they can be seen (greyed out) in disk utility.

Sep 3, 2019 12:22 PM in response to BDAqua

It looks like the linux article will help a lot, but not directly (because the mount command simply re-asserts the specification of etc/fstab). But it does give me a good handle on how to do some research in the unix man files to see how to do what I want. For example, if I start up with a TM backup partition not mounted, in theory I could execute some script that would turn the 'noauto' record in the fstab file into a comment, then execute the unix mount command. However that kludgy approach could yield some tragic unintended consequences ( a la Mickey Mouse in the Sorcerer's Apprentice) ... but I think you've pointed me toward the right kind of magic. It'll take me awhile to do the research in the unix man files.


One factoid I'd like to emphasize to anyone who pays attention to this thread: before APFS was introduced, it was possible for me to use Disk Utility to mount a partition from an external USB drive that had not been mounted at start-up. But after I upgraded to High Sierra (which uses APFS) I could no longer do this even though the High Sierra Disk Utility shows the unmounted partition to be present (but greyed out) on the external drive). This implies that APFS treats HFS+ partitions not mounted at start up differently than AFS did in prior MacOS versions. (Ironically, when an HFS+ partition is mounted at startup, it can be unmounted and remounted by Disk Utility)

Sep 3, 2019 1:00 PM in response to fiddleaway

In order to get a better idea of your setup can you run a couple of Terminal commands in order to see what exactly your mac thinks it has connected to it (as far as devices and volumes):


diskutil list

and specifically for the drive having problems (assuming in this example the device is disk5 -- change for your setup if needed):


diskutil info disk5


If any of the output contains personal info -- please modify it and tell us what exactly you changed, e.g. "changed name of mount point to abcedfg" or something along that line...


Good luck...

Sep 3, 2019 4:14 PM in response to BDAqua

Doh! I should'a thought of that !!! Winner, winner, chicken dinner! Short of a solution that doesn't require writing script, this really solves my problem, only thing that would be better would be some action already built into utilities delivered with 10.13.6 (like Disk Utility) that simply is there to let one do what you used to be able to do with Disk Utility prior to 10.13.6.

Sep 5, 2019 1:04 PM in response to BDAqua

Eureka !!! - all I really had to do was add some commas to my original fstab file !!! This problem was caused by operator (me) error!


When playing around with creating a couple of versions of fstab, as BDAqua suggested, I noticed that my original fstab had an error in the mount options field (fs_mntops) of the fstab commands which were meant to bar certain partitions from being mounted. The two options given were not separated by a comma: rwnoauto (wrong), instead of rw,noauto (right). These erroneous commands succeeded in barring the partitions from being loaded (while still being visible to Disk Utility after startup), but prevented them from being mountable later by Disk Utility. All I did was modify the fstab file by adding a comma to the options, and now I can bar these partitions from being mounted at startup .... but still use Disk Utility to mount them on the rare occasions I want to use them later. ..... Bingo! fixed!


So, this problem is due to operator error (although my previous OS (Yosemite) didn't exhibit the problem - clearly something subtle has changed regarding the handling of fstab commands in High Sierra. I'm calling this post the solution to help future readers, but really BDAqua's Terminal suggestions are what led me to realize my long-standing configuration errors.

High Sierra: won't mount external volume not mounted at start-up

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