Thank you very much for reporting this solution here.

I posted an issue in the following post almost two years ago. I would like to share a solution because solution suggested in the following link is not correct.

ZDB Backup | Error description : [183] Cannot create a file when that file already exists. - Data Protector Discussions - OpenText Data Protector (microfocus.com) 

When you backup LUN mount points via ZDB, DP spits out following error when attempting to backup LUN mounted under the Root volume.

Details of backup specification is shown below.

Inside the node, following mount points are available.

Inside F: Root mount point, there are following mounted LUNs.

This error goes away when you uncheck the Root mount point as shown below.

Also note that OpenText support spent almost 13 months to investigate and diagnose this issue. I tried to replicate the issue in the lab environment and found the solution.