Just upgraded to Monterey this weekend and noticed almost this exact behaviour. For me it was over 372 GB of "System Data" and my local storage went from around 1.3 TB to 870 seemingly overnight.
I know that I didn't use this much before because I have a clone of my hard drive pre-upgrade that only used about 859 GB.
I did manage to get back close to 100 GB just by deleting local Time Machine snapshots but something is definitely amuck. My twitter thread here has a bunch of other details:
Spoke to Apple Support which suggested re-indexing my hard drive but that did nothing.
I did some more digging online which suggested using Daisy Disk to find large files. I did exactly that to "purge" over 400 GB of "purgeable" data which got things looking a bit more normal again. Although I discovered that, apparently, one of the things that is purgeable is local copies of my Photo Library because that shrunk like crazy.
However, "System Data" was still over 100 GBs. So I followed the advice of the senior advisor I spoke to from Apple Care and reinstalled the OS from recovery mode.
And... nothing seems to have changed. Still have a massive amount of "System Data" (125 GB) so I honestly don't know what else to do at this point. The Senior Advisor said I could use time machine to roll back to Big Sur and I am tempted to do exactly that, but not sure it's worth the hassle?
Also tempted to just wipe the machine, install Monterey clean, and use migration assistant, but even then, that might carry over the bad system data so really not sure what my best option is anymore.
Running Monterey 12.3.1 on a MBP 16-inch 2019
2.4 Ghz 8-core i9
32 GB ram
I know that I didn't use this much before because I have a clone of my hard drive pre-upgrade that only used about 859 GB.
I did manage to get back close to 100 GB just by deleting local Time Machine snapshots but something is definitely amuck. My twitter thread here has a bunch of other details:
Spoke to Apple Support which suggested re-indexing my hard drive but that did nothing.
I did some more digging online which suggested using Daisy Disk to find large files. I did exactly that to "purge" over 400 GB of "purgeable" data which got things looking a bit more normal again. Although I discovered that, apparently, one of the things that is purgeable is local copies of my Photo Library because that shrunk like crazy.
However, "System Data" was still over 100 GBs. So I followed the advice of the senior advisor I spoke to from Apple Care and reinstalled the OS from recovery mode.
And... nothing seems to have changed. Still have a massive amount of "System Data" (125 GB) so I honestly don't know what else to do at this point. The Senior Advisor said I could use time machine to roll back to Big Sur and I am tempted to do exactly that, but not sure it's worth the hassle?
Also tempted to just wipe the machine, install Monterey clean, and use migration assistant, but even then, that might carry over the bad system data so really not sure what my best option is anymore.
Running Monterey 12.3.1 on a MBP 16-inch 2019
2.4 Ghz 8-core i9
32 GB ram