Hi.... I was writing the query below (which I think this should be an easy one to solve) but for the life of me I can't find a solution by trial and error or looking at previous forum entries. It seems I have a similar situation to the OP's, except I am using LR5.6. Great minds think alike, or perhaps encounter similar problems.
All my photos and associated Lightroom files (catalogue, sidecar files, thumbnails, etc) are on a portable HDD.
I have 2 computers- a laptop which I travel with and a desktop with a good monitor etc which I use as my "main" computer. The laptop runs on Win 8.1, 64 bit, and the desktop runs on Win 8.0, 64 bit.
The laptop has one drive, an SSD labelled "C". The desktop has two HDDs, labelled "C" (for the programs and OS), and "D" (for data, but excluding photos because they are all on the portable HDD.)
The laptop correctly recognises the location of the photos, catalogue etc when I plug it in. It always labels the portable HDD "I". (No idea why it skipped the intervening letters!)
When I plug the portable HDD into the desktop, Lightroom does not recognise the drive. It shows the folders (correctly named), thumbnail images etc as being located in drive "D" (where they are NOT), but not the originals. The "Folders" panel in the Library module shows question marks in front of all the folder names, I assume because they are not on drive "D" in the first place.
Windows Explorer recognises the portable drive with my photos correctly (and the photos are in fact there, intact and uncorrupted).
Using the menu command File>Open catalog and navigating to the portable HDD, and selecting the catalog (I only use one catalog), does not result in the catalog opening in Lightroom- it just shows "D" again with the thumbnails but no originals. I have also reinstalled Lightroom (v5.6) a couple of times but that did not solve the problem. It is unable to export any photos on the desktop machine, as the sources files are (it thinks) "offline or missing".
Any ideas o esteemed Lightroomers?