Re: Safest way to preserve 100+gig of data when upgrading
The "safest" way -- is to NOT have those files on the drive when you do the reinstall. Since you're not up to inserting 100 or so DVDs, the way to do this is to migrate all those files to an external drive.
A second way (not so safe, more work, but cheaper -- if you don't have an external drive) involves the following:
1) Boot into Ubuntu desktop CD and use GParted to (1) shrink the Ubuntu partition, and (2) create a new partition on the drive.
2) Migrate some of those files to the new partition.
... continue to shrink the OS partition, expand the data partition, until you have all the files migrated to the new partition. NOTE: That with 300GB of files to migrate, this is going to take a LONG time -- hours if not days.
Then, when you reinstall, use the "something else" option to reuse the existing Ubuntu OS partition. Be VERY CAREFUL to not format or overwrite the other (data) partition.
Ubuntu 16.10 Mate, Mint 18 Mate; MS Win 8.1, MS Win10 Pro.
Will not respond to PM requests for support -- use the forums.