OST files by default are saved in appdata\local\microsoft\outlook. The environment I have been working with is plagued with the following error:
The 200 user environment had elected to use Persona Management to roam the local profile instead of using persistent disks. By creating a new pool with persistent disks and migrating the users the issue is fixed.
Other issues manifested from local roaming. Ntuser.dat was being written to the VM at user log on but not being written back to the network location after sign off. This would cause any changes made to HKCU (current user registry hive) to be overwritten. My initial fix was to force outlook to populate OST files to the user’s home directory instead of the default directory in appdata. This only applies to newly created profiles, so I had to delete and recereate their mail profiles. All changes were lost after the user re-logged in, as user mail profile data is stored in HKCU.
Persistent disk seems to be the way to go and fixed all of the problems. However, I did not chase down why local was not properly writing back to the network at user log off. I reached out for more information but could not track down the underlying issue. Anyone with an idea, please assuage my curiosity and let me know. Are you using local roaming with Persona Management in your environment?