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

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)

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.

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.

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)

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.

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.