Today, on MacOS 12.2.1, I had to migrate from one MBP 14" to another (needed more RAM so bought an entirely new MBP - same 500GB SSD). This is how much space I was using:
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But when I used Migration Assistant, this is what it told me:
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~/Library is 467.42GB in size? No way.
After a ton of searching for this issue I couldn't really find anything solid. Terminal tools like `du` and freeware tool GrandPerspective both said "Nah, bro, ~/Library only has like 30GB of stuff in it.
It was only on a hunch that I thought "huh... you know what is about 400GB? My OneDrive account!"
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So. To try this out I unlinked my MacBook from OneDrive. Deleted the OneDrive directory. Uninstalled OneDrive. And even, out of an abundance of caution, deleted a 12GB directory I found within ~/Library that had OneDrive's name on it.
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Started up Migration Assistant again. And while the calculated account was still inflated, it was far less inflated (165GB vs 467GB) and I was able to Migrate to the new MacBook!
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And yes, even that 165GB was a gross over-calculation. The new MacBook's used SSD was just about identical after Migration.
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This is a really scary issue because I tried using my TimeMachine backup as a migration vector. To restore from TimeMachine would've been impossible with this inflated size. And I'll say that I only have 500GB of space for my TimeMachine hard drive backup - so these ghost files aren't actually taking up space on the TimeMachine backup. MacOS is simply scaring itself out of migrating/restoring from inflated Measurements.