Clarification on ERROR_ACCESS_DENIED and admin rights

[zomp]

FreeFileSync 14.9 on Windows

I keep getting ERROR_ACCESS_DENIED for files that need to be moved/deleted on the target side when backing up my files.
For some reason, those files always turn out to need admin rights to be deleted/moved.
They are normal files like images or documents in the usual Pictures/Documents folders. No program files or any other "restricted folders".
I use an external HDD and all files on there have been created by using FFS.
If it's only a few files I usually just delete them manually. If it's more, I need to restart FFS as admin and run the whole comparison again which can take quite a while. (I guess technically I could just select the affected folders to reduce the time)

I'm wondering 2 things:
1. how come some files need admin rights to be deleted? I created them on the same computer with the same account and haven't changed any permissions on the files/folders or the like.

2. if I run FFS as admin and synchronise files – will those new files all need admin rights again to be deleted/moved in the future? Or does it have no effect on the file permissions if FFS is run as admin?

[xCSxXenon]

1. Permissions on the files
2. Yes, probably?

I would check permissions for files that do work and apply them to the entire drive recursively

[Hemps]

Not sure if related or might assist someone else.

I cut down on FreeFileSync errors - before first sync I make sure to right click drive letter I'll make the destination - Properties - Security - Advanced
Then add FULL control
System
Administrators (Local pc)
Account i'll use

Then make sure Replace all child objects...... box is ticked.

My Updates/Mirrors run every time - syncing daily around 5million files