What is this device?

It either a TimeCapsule or Airport extreme. If its a Time capsule you can remove the drive and use the drive, they come in 1, 2, or 3TB sizes. Extremes have no drives. Otherwise its a NAS type device and you can share the internal drive on the network as you would any NAS type device. Its not a device you plug in a machine USB port to use as a DAS.... unless you want to use ethernet cable connection.

sim667 said:

I couldn't work out how to mount it. I gave up.

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Here's a support article that would help you use the Time Capsule hard drive.
Keep in mind that the Time Capsule, at its heart, is a wireless router, so the internal, or maybe an external USB drive attached to the Time Capsule, are shared storage, available through the router.

sim667 said:

I couldn't work out how to mount it. I gave up.

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That is probably for the best. That device is not the best tool for your task. Whether it is an AirPort Extreme Base Station (AEBS) or the even more capable Time Capsule that thing is primarily a networking and networked storage tool. Read the model number off of the router. The A1521 AEBS has AC WiFi and a Gigabit speed network router. The A1470 Time Capsule has the AEBS features plus an internal hard disk.

With the MacBook Pro that you have, I would be inclined to obtain and install fast external storage by using the Thunderbolt or USB 3.0 ports. Then there would be no need to fiddle with or dismantle that valuable WiFi router.