Can't Delete Trash, Error Code -8072

[GS Owner]

To understand things, you have to understand that come the weekend I switch to an external HD.
This HD is where I keep a near exact copy of System I use on the iMac. In case some driver is corrupted or anything happens on the Main HD, I have the ExternalDrive as the Ultimate Backup. Some devices also work better with the External's OS than the usual iMac HD OS.

The External is also where I keep the Time Machine backups.

The start of the month. Switch to the External. Run the virus protection programs and general deletion of old and obsolete files. Oldest Time Machine backups to trash. Backups for April 4, goodbye.

But they wouldn't delete. I would get Error Code -8072.

Eventually I would go the the Main OS and delete every other Time Machine backups, but NEVER the External HD's trash. I've been going online and tried to find a solution to the problem. So far no luck.
I'm very hesitant to using a spare copy of re-installing El Captain for obvious reasons.

Until I can afford to replace this particular iMac I'm stuck with OS 11. What can I do?

 

[GS Owner]

Addendum: I've tried Trash It! with no results.
Somehow while in the Trash some files are still being used.
HOW?

 

[DeltaMac]

You may be able to trash those "in use" files after disabling SIP. You have to boot to your recovery system, then run

csrutil disable

in the terminal. The only way to do that is from the terminal when booted to your recovery system. Then, restart to your normal system. Empty the trash. When that is successful, restart to your recovery system and re-enable SIP. Same process in the terminal, but change "disable" to "enable"
recovery system Terminal:

csrutil enable

 

[GS Owner]

My mistake.

I tried it and It Worked. 150,000+ useless files gone.

csrutil disable Worked!