Tried the Application Pool trick listed here, but it still failed.

https://social.technet.microsoft.com/Forums/en-US/b2274a17-13c0-4598-82cf-2724bc427251/error-downloadcontentfiles-failed-with-hr0x80070193?forum=configmgrgeneral

@BBigford said:

I've been looking for the GUID entries in SCCM 2012 but have been unable to find them. When a machine is reimaged by SCCM, I was under the impression that it does indeed log the machines entry in SCCM. Google has evaded me... do you know where they get stored at in 2012?

2012 is different to 2007 in that you don't need to go hunting for the GUID unless you are digging through the database.

That being said...

If you go to Assets and Compliance --> Overview --> Devices, you can add the GUID column by right clicking any of the column titles and selecting ID. Deleting the device from here also deletes it from the database.