A Microsoft file hosting and synchronization service.
Dear Zachary,
Good day! Thank you for posting in Microsoft Community.
As per your description, it seems that you are trying to transfer files from Dropbox to OneDrive via OneDrive sync client for almost 50 GB and just over 20,000 files, however, stuck at "Preparing to Upload" for the past 3 days. And the folder shared by other has been added as" shortcut to my files", however, not showing up in OneDrive sync folder.
If my understanding is right, firstly, we'd recommend you check the operating system version that you are using. If it is Windows 7, 8, 8 and 8.1 and below, please refer to OneDrive sync app no longer supported on Windows 7, 8 or 8.1 to Update System.
If the operating system meets the requirement, please make sure that there is sufficient free space on your hard drive so that OneDrive can run properly. We'd recommend you pause sync by tapping OneDrive cloud > Help & Settings > click Pause syncing > select a pause option, wait a few moments for OneDrive sync to stop, tap OneDrive Help & Settings, click Resume syncing to start OneDrive uploading, and uncheck File on Demand to check the outcome.
For your reference: How to Pause and Resume sync in OneDrive - Microsoft Support
Sync files with Files On-Demand - Microsoft Support
If the issue persists, we'd recommend you unlink OneDrive, reset OneDrive and then re-setup sync in another location to check the outcome.
It is also recommended to uninstall, reinstall OneDrive and update OneDrive to the latest version.
Additionally, there are some limitations and restrictions for OneDrive sync listed in this article, please make sure that file path not too long, restricted characters not included in, the invalid file type not being synced and the total file count in OneDrive should not exceed 100,000 files.
If the issue still remains, we're afraid that this issue may need to check the environment on your side for further investigation.
As forum support, we may not have enough permission and resource to directly check the environment on your side.
In this situation, we sincerely recommend you contact your Microsoft 365 administrator, follow steps in this article and try to create a support ticket to contact Online Support. The support team over there has higher permission than us, they can remotely help check the situation on your side, involve more resource to collect logs and help you do the troubleshooting steps. This would be a more efficient way in handling this case for you.
For reference: How do I find my Microsoft 365 admin?
If the scenario above is not consistent with yours, you can also post back and point that.
Thanks for your cooperation and understanding! I hope you keep safe and healthy always.
Sincerely,
Rhoda | Microsoft Community Moderator