Mac mini using 7000 GB of data in a week — can’t find what’s downloading

Ran into a similar situation with mds_stores doing 100's of GB of writes per day but was not using disk space as it looked like it was just writing the same file over and over. This one was devious in that it never impacted performance and was only found when I was doing one of my periodic disk health checks. I actually narrowed it down to a single directory that for some reason Spotlight decided was changing constantly even though after monitoring the directory zero changes were taking place. I dragged that folder into the privacy pane and the writes stopped.

A subsequent macOS update did resolve the issue.

There was also a case where the database for Spotlight somehow got corrupted and was actually consuming disk space until I had a few 10's gig left (originally had ~300GB free on a 512 GB drive). One can wipe and reset Spotlight for a volume via terminal command ("sudo mdutil -E / " for the boot volume or the general for of any volume "sudo mdutil -E /Volumes/OtherHardDrive").

Got sidetracked.

It should be easy to find the culprit if it is network downloads by selecting show all processes in the menu and then simply sorting by clicking on the Recvd bytes header in the Network pane of Activity Monitor.

Consider downloading EtreCheck and posting its report in a reply to this Discussion.

Instructions:

How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report - Apple Community

I had a similar problem a year ago.

According to Activity Monitor my mini was writing about 100GB per day although there was no sign of it on the SSD.

I tracked it down to Spotlight and launchd (a Spotlight helper) so I deactivated Spotlight and the daily writes were down to around 10GB which is normal.

To deactivate Spotlight etc. open System Settings>Spotlight and scroll to the bottom right where you will find Search Privacy . . .

Click it and in the window that opens add every drive you connect to your Mac.

Correction: It's mds_stores not launchd that appears to do all the writing.

If I am not mistaken OP is concerned about network data usage, not local r/w activity.

EtreCheck will help resolve any confusion.