Not sure what the problem here is but
####3-MacBook-Pro-2:~ ###$ /Volumes/###\'s\ Overkill/SteamLibrary/steamapps/common/XCom-Enemy-Unknown/XCOMData/XComGame/xew/install-LW-OSX.sh ; exit;
find: /Volumes/### Overkill/SteamLibrary/steamapps/common/XCom-Enemy-Unknown/XCOMData/XComGame/xew/install-files/: No such file or directory
Installing Long War for XCOM:EW, please, be patient...
Can not find game files. Please specify xew dir location
/Volumes/###\'s\ Overkill/SteamLibrary/steamapps/common/XCom-Enemy-Unknown/XCOMData/XComGame/xew
User files dir: /Users/####/Library/Application Support/Feral Interactive/XCOM Enemy Unknown/XEW
old backup already exists in /Users/###/Library/Application Support/Feral Interactive/XCOM Enemy Unknown/XEW/backup or /Volumes/### Overkill/SteamLibrary/steamapps/common/XCom-Enemy-Unknown/XCOMData/XComGame/xew/backup.
Owerwrite old backup? (y/n) y
Removing old backup and all its content...
Wrote backup to:
User files: /Users/###/Library/Application Support/Feral Interactive/XCOM Enemy Unknown/XEW/backup
Game files: /Volumes/###Overkill/SteamLibrary/steamapps/common/XCom-Enemy-Unknown/XCOMData/XComGame/xew/backup
Copying install files...
cp: directory /Volumes/### Overkill/SteamLibrary/steamapps/common/XCom-Enemy-Unknown/XCOMData/XComGame/xew/XComData/XEW does not exist
logout
Saving session...
...copying shared history...
...saving history...truncating history files...
...completed.
[Process completed]
Is the terminal log I think its todo with installing on an external drive