airport extreme usb harddisk doesn't show, on discoect I get notified, other NAS drives are acessible

I want to coect two encrypted usb drives to an airport extreme. Neither does show up in preferences or finder. The password of the harddisk is accepted by the airport. Another NAS in the same Wlan is acessible from my computer. If I unplug the usb drive from the airport, the airport asks me for the drive's password and accepts the correct password.

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