iTunes Backup Still Using C Drive After I Changed Directory

See the 'Find backups stored on your Mac or PC' section in:

Locate backups of your iPhone, iPad, and iPod touch - Apple Support

"Though you can copy the Backup folder, you should never move it to a different folder, external drive, or network drive."

See Relocate iOS device backups. Review the rest of the Make a split library portable user tip for advice on organizing the library on secondary or external storage. Once the backup data has been relocated in this fashion it still appears to be on the C: drive, because that is the path iTunes will continue to use, but files stored in the folder actually live in the alternate location you've linked to. Note that iTunes still checks for free space on drive C: so if your backup is going to take 20Gb of storage you must have that free on the C: drive even though the data ends up on D:.

tt2

What if our backup is 200 gb but our c: is only 110 gb? I was able to make a 185 gb backup before, but now it does the check and says no space available even though I have 300+ gb free on the drive I'm trying to back up to. It looks like the newer iTunes version checks and maybe an older one didnt...

In which case I'm surprised that it worked before. Time for a bigger system drive, or give up and use iCloud for backup? 😒

tt2