My friend and I were discussing partitioning and we disagreed on repartitioning when the drive was full.

How full is too full before partitioning will mess up your file system?
I've partitioned before but it was only after the OS was installed, so only about %5 of the drive is being used. I read that 90% or more is too much.

My friend claims it doesn't matter, and you can repartition even if the drive is 95%+ full.

Thank you for any help or input you can contribute to this.

There are 2 definition of "full" in the context.

If you are talking about creating, deleting, or re-sizing partitions, then a full disk means the all the space is assigned to partitions. Think of it as a large land parcel being subdivided.

"Full" within a partition means there is no more room to store data. Of course, if a partition is full, then it cannot be re-sized down to make room on the disk.

In principle, as long as any partition has empty space, then you can move things around to make other partitions bigger--or to create more. In practice, it depends on the exact configuration.