Import from another catalog does not allow me to select folders

I am in the process of merging multiple LrC catalogs into a single master catalog and consolidate/move all the photos to a new large disk. I learned this process from a workshop and successfully done two of such operations. Here are the steps I took:

1. Open the catalog in LrC of the one I want to merge into the single catalog;

Select All Photographs; then in "edit" choose Select None; then in "File" choose "Export as catalog" and select "Export negative files" and "Include available preview"; then enter the location where I want to exported catalog file etc.

This operation will generate all the folders in the LrC catalog to be exported and the catalog files.

Once this operation completes, to merge into the single master catalog:

2. Open the single master catalog in LrC:

In "File" select Import from another catalog"; then in the popup, navigate to the folder where the exported catalog and folders reside, select the catalog file AND ALL the folders; then enter the location where you want to move all the photo files to;

This operation worked well for my first two merges which contained a small number of photos (about 19k). But for my most recent attempt, the export worked successfully but I got a popup message saying it was not able to locate a large number of photos (about 23k out of a total of 46k) which did not surprise me because there were a couple of USB external drives might be corrupted/old. But the operation was successful and I see the generated .lrcat files in the export location AND all the folders.

Now I tried to perform the import operation. But LrC would not allow me to select multiple items - I could only select ONE item, either the catalog file, or any one folder. I went to the OS Finder and was able to make multiple selections in that location. 

What happened? Is it because I have too many items in that location - I have more than 20 folders in addition the the LrC catalog related files. Or is it because I have missing photos? That really should not matter because I plan to resolve the missing files issues later in LrC.

Please help!!