I assume this the home version. Some causes slow scan
1. 27million files cripes all zip and avi maybe . Zip files slow down the scanning the program has to open the file and scan the files that were zipped. Large zipped files are very bad as are large avi and game exe files.
2. As you are finding issues best to let it go on. In sceduled scan you can exclude folders and files from being scanned so after the external drive is scanned add the drive to the exclusion list as well as folder with files that are never changed ie photo albums etc. this might help you next time.
3. Ensure use minimal resources not selected .
Will mention re the forum for Mac to our admin.