"Unable to connect to C: or it does not exist"
Here are some workarounds you can try:
1. Close Windows Disk Management console. It's using Logical Disk Management service which extpart.exe is using.
2. Make sure there are no snapshots
3. Stop some non-essential services i.e:
- Windows Search
- File Replication service
- DFS Replication service
- Perfect disk PDEngine service