Hi VietHuynh3,
Thank you for reaching out to Microsoft Community.
For the C: drive of the Surface Book 3, please run these steps to troubleshoot the issue and check you will then be able to access the files on the C: drive:
1: Run Surface Diagnostic Toolkit
2: Update drivers and firmware
Check what is the current OS Build of your Windows version by going to Settings > System > About.
Go to Download driver and firmware for Surface, choose Surface Book 3 then click Download on the next page.
Choose the .msi file that matches your current OS Build and click Next.
After downloading, run and install the .msi then Restart.
3: Run System File Checker Tool in Windows 10
For the external SSD drive (Samsung T7 SSD) that you have, Have you tried disconnecting it while the device is off then plug it back in by the time the Surface Book 3 is on?
If this answer your question, we highly encourage you to inform our Community by clicking on Yes or marking my post as an answer. This will help other members who may have the same issue as you. Otherwise, don't hesitate to update us so we can further assist you.
Kind Regards,
Marrion Y.