You can do it by using [Database management].
This function enables you to move library database to an external HDD and also enables you to switch library.
Before doing the task, I recommend you to [Backup library] in [File] menu for safety.
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Move first library database to your first external HDD
- Connect your first external HDD to your PC/Mac
- Run rekordbox and open [Preference] > [Database] > [Advanced] > [Database] tab.
- Click [Move Database] button on Database management.
- Select the first external HDD and click [OK]
- Wait for moving process. This takes for long time. For me, 1 minutes for 100 tracks.
- After the process, click [OK] and now your first library database is in your first HDD
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Create second library from scratch
- Close rekordbox.
- Detach your first external HDD from your PC/Mac.
- Run rekordbox and [Cannot find Master Database...] is displayed, click [Yes] to create second library.
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Switch library database
- Close rekordbox.
- Connect the first HDD
- Run rekordbox and open [Preference] > [Database] > [Advanced] > [Database] tab.
- Select your first HDD to use first library, or default HDD to use second library.