To reproduce:
1. Install Unity (2018.1.0b11+ or 2018.2.0a7+) on Mac on external drive using Download Assistant.
2. Create a new project or open existing.

Actual result: Editor cannot be opened with an error message "Fail to start Unity Package Manager" (see screenshot attached).

[Package Manager] Server::Start -- Port 61558 was selected
Launching external process: /Applications/Unity 2018.2.0a7/Unity.app/Contents/Resources/PackageManager/Server/upm-macos
[Package Manager] Server::Start -- Cannot start server process [path: `/Applications/Unity 2018.2.0a7/Unity.app/Contents/Resources/PackageManager/Server/upm-macos`]
[Package Manager] Failed to start Unity Package Manager

Permissions to file:
-rw------- 1 asyasmirnova staff 76224776 Mar 27 02:41 /Volumes/Transcend/Applications/Unity/Unity.app/Contents/Resources/PackageManager/Server/upm-macos

Expected result: Editor opens with no issues.

Permissions to file:
-rwxrwxr-x 1 root admin 76224776 Mar 27 02:41 /Applications/Unity/Unity.app/Contents/Resources/PackageManager/Server/upm-macos

Workaround: Add execute and read permissions for "Unity.app/Contents/Resources/PackageManager/Server/upm-macos" and "Unity.app/Contents/Tools/" directories. '.

Issue presents since:
2018.1.0b11 and 2018.2.0a7