How to clear 217Gb of <Unknown> space on my C drive?

Most disk space management tools do not list alternate data streams, do not properly calculate hard-linked files and do not list the space in folders that you do not have access to, e.g. the folder "System volume information" folder in the drive's root. Without a tool that does all this correctly, you won't be able to get correct results for directory sizes. TreeSize Professional does this job once you activated the option "Track NTFS alternate data stream and hardlinks" in its options and have started it as administrator. Full disclosure: I am one of the developers of the TreeSize product family. A free 30 day trial is available.

In rare cases the structure on the volume may be damaged. in this case CHKDSK /F /SCAN /SPOTFIX might help.

In case the "System Volume Information" folder seem to be unreasonably large, you may reduce the space that Windows may use for shadow copies using this command: vssadmin resize shadowstorage /For=C: /On=C: /MaxSize=1GB. In this example, the space is set to 1 GB. As a quick solution you may execute this command: vssadmin Delete Shadows /For=C: /Oldest. It will delete the oldest shadow copies.

For both commands you need to run CMD.EXE as administrator.