Sierra does not show mounted disks in Finder

I do not know if this helps in diagnosing the problem, but the Finder did display my laptop's hard drive when I hooked it up in transfer mode. However, it still refuses to show the attached external back-up drive.

Understood. The disks in question did not display in either location, the desktop or within the sidebar of Finder windows. Before upgrading from Yosemite to Sierra, I had both the external drive and a Flash drive mounted. After Sierra was installed, both were missing on the desktop. I opened a Finder window; both were missing from the Sidebar. I opened Finder preferences, making sure that in both the General tab and the Sidebar tab that the displaying of hard drives box was checked. This did not restore visibility. Tuesday night when this first happened, I was only able to get both displayed on the desktop after opening TinkerTool and clicking on the box to display hidden files. Then yesterday afternoon, I inserted a flash drive into a USB port to transfer over files before going to work. The flash drive did not display on the desktop once again. The external hard drive also was no longer on the desktop. (Incidentally, through all of this, the computer's own hard drive was always displayed.) I went to Finder preferences; under both the General tab and the Sidebar tab the appropriate boxes were checked. I opened Disk Utility; both devices were mounted. I unmounted and remounted them, hoping they would appear on either the desktop or in an open Finder window. Neither occurred. I tried the step from Tuesday night, opening TinkerTool and displaying hidden files. This time, the step did not work. At this point, I wrote a note to this forum after a quick search showed me that no one else had apparently encountered this problem. I then hooked up my laptop in Transfer mode to see if that would display on the desktop. It did. When opening a Finder window to transfer files, I noticed that now, the attached external hard drive was displayed in the sidebar, but still was not on the desktop. Transferring the files I needed, I disconnected and left for work. Upon returning and seeing your first message, I re-checked Finder preferences under both tabs. Both remained checked. The external hard drive remained present in a Finder window, but absent from the desktop. I inserted a flash drive; this displayed normally. Puzzled, I decided to use TechTool Pro to do a volume rebuild on my external drive; when this process finished the drive was now visible on the desktop. As of this writing, all remains visible.

Just restarted my computer. Once again, the external hard drive was not on the desktop, and was not displayed in a Finder window. I checked my Finder preferences for both window sidebars and the desktop. The preferences remained unchanged. This time, I was able to restore the external drive to the desktop by using TinkerTool's display hidden files feature. Am I truly the only one having this issue after upgrading to Sierra?

Yes. Two days ago I even had a DVD that I inserted into the CD/DVD drive not show up on the desktop. While my external drive has remained visible through multiple restarts over the past week, I still encounter occasional bouts of inserted flash drives not appearing. I've tried different USB ports per BRLM28's experience; so far that seems to have helped. When drives or DVD's have not appeared this past week, TinkerTool's show hidden files option has succeeded in making them visible.