Problem extracting / backing up hard drive data from Precision 3520

My new Precision 3520 mobile workstation is down after only 3 days of use, OS won't start normally. I decided to return it anyway.

So I need to back up hard drive data first. But new problems prevented me from doing it...

Info on my Precision 3520:
OS: Ubuntu 16.04
HDD: Seagate ST500LM021, 500GB 7200RPM

What I did:
Take out the Precision 3520 hard drive, and connect it as a USB device to my old Inspiron N5050 laptop (runs windows 7).
The connection is done via an Insignia SATA-to-USB 3.0 converter, it worked fine with other HDD of comparable model (a Seagate Momentus 5400.6, 500GB 5400RPM).

Problem:
Upon connection, the driver is successfully installed, but the HDD shows up as having only 2GB size, with only OS files inside.
The rest 400+GB of disk, with user data, can't be found.

How to solve this?

Just in case if you need this -- the Precision 3520 can't enter OS desktop.
After displaying the Ubuntu start up logo, it got stuck there with black screen, keyboard not responsive, but the following error message pops on and off once every 2-3 seconds:
[***:*****] pcieport 0000:00:1c.0: PCIe bus error: severity=corrected, type=data link layer, id=00e0(transmitter ID)
[***:*****] pcieport 0000:00:1c.0: device [8086:a111] error status/mask=00001000/00002000
[***:*****] pcieport 0000:00:1c.0: [12] Replay timer timeout
(Note: those *** are some random numbers)

Running the pre-boot hardware diagnosis shows all devices are normal.
And it can still enter BIOS setting.

After clearing "power event log" in BIOS setting, the error message above disappeared, but the same problem persists.
(Now I'm not concerned about this OS start up problem, because I'll return it anyway. What I want to solve is the HDD data backup issue!)

I appreciate anyone who can help me on it!