Libraries stored in portable storages cannot be accessed by both Windows and MacOS

The Eagle library itself supports macOS/Windows cross-platform use. If you can't open the library on both macOS and Windows, it could be that the hard disk format is not compatible for both OS. For example, storages formatted in NTFS are not compatible with the macOS.

How to fix the issue?

1. Use the storage that is both compatible with macOS/Windows, such as FAT32 or exFAT (recommend)
2. Use a third-party plugin to make macOS support NTFS format (not recommend, refer to this article)