Hey there,
Our games, on Windows, were made for NTFS file format. I noticed in your post you mentioned using this external SSD for the game. The exFAT file format is going to be the cause of this issue.
The resolution would be to either reformat the device as NTFS or create a partition. This can be done using built-in Windows tools. For information on formatting drives please reach out to Microsoft.
Note, please back-up any important data from the drive before changing formatting or partitions.
I also highly recommend deleting the current install for World of Warcraft and conducting a clean install to the desired folder after resolving the exFAT file format issue. This will help ensure all the files are fresh and in-tact.