Merge to HDR not Working in Lightroom Classic 10

Thanks. That worked.

Make a backup of the current catalog. Then right-click the empty folder and do Remove. The folder and its parent drive should disappear from LR.  If it warns you that the folder isn't empty, click cancel.

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So, went through the entire process and got all the folders/files back on to one drive. However, the second drive remains, showing an empty folder that has the same name as the drive, except that the folder is named ThunderBay #1 Raid 5, instead of Thunderbay #1 Raid 5. There is no corresponding folder with this name in the drive containing all the files.

No need to apologize, this is a mess created by bugs in LR.

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Of course. Duh. Sorry.

If you right (or control) click on the folder names, you can have it show the "parent folder". Sometimes these problems suddenly resolve themselves if you do this and Classic suddenly realizes it already has that somewhere else.

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In the second instance of "Thuderbay #1-Raid 5", right-click one of the folders and do Show Parent Folder.  Repeat that process until you end up with a single top-level folder "Thuderbay #1-Raid 5". Now both instances of the drive have the same top-level folder, also named "Thunderbay #1-Raid 5".  It should look analagous to this:

You can use either article's recipe (which are essentially the same) to get rid of second entry for the drive/top-level folder.   Note the Adobe article calls out that if LR asks if it's ok to merge, you answer yes.  Also note that a comment to the Lightroom Queen article is from a Windows user who used the recipe to merge the entry for a "w:" drive with a "W:" drive.

Before starting, make sure to make a backup of the catalog!  Make two backups just to be sure!  These procedures don't move folders and photos around on disk, they just change the catalog. So if you make a mistake or it doesn't go according to the recipe, you can simply restore your backup.

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The process you cite is for duplicate folders. What I have is duplicate drives with different contents. Here is a suggested process for dealing with that, which is similar, but not exactly the same: https://helpx.adobe.com/lightroom-classic/kb/lightroom-displays-same-hard-drive-twice.html  The problem is that there are no top-level folders in the duplicate drive, as there are in the proper drive, only loose subfolders. I am not sure how to proceed with the situation I have and am afraid I might screw something up.

The names look exactly the same to me in your screenshot. Pretty sure this is the duplicate drive bug. I've run into this myself and it is quite annoying. I would caution against using special characters by the way. The hashmark in the drive name might cause issues.