Are you unable to manually copy the CONTENTS folder from the USB drive to computer B? As long as the song / grid / hot cues are imported to the library, you could always relocate the songs manually.
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tl;dr:
Screenshots are attached for relevant devices, etc. Using Rekordbox v6.5.2 on both machines. There are a bunch of other issues that went along with this process, but I'm trying to avoid bogging down the post w/extraneous details.
Origin machine (A)

USB storage

Destination machine (B)

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Are you unable to manually copy the CONTENTS folder from the USB drive to computer B? As long as the song / grid / hot cues are imported to the library, you could always relocate the songs manually.
...one thought that just came to mind, have you given rekordbox permission to access removable devices in the System Preferences > Security & Privacy > Privacy?