This is drive E:, a btrfs file system. Rustbelt reads it with its own
driver (src/fs/btrfs.rs) but never writes to it: btrfs is read-only here.

btrfs keeps everything in copy-on-write B-trees. A file's bytes live in
"extents": small files (like this one) sit right inside the tree as
inline extents, bigger ones in data blocks elsewhere on the disk. This
image was made with --compress zlib, so most extents are compressed.

Things to try:
  ls -l E:/            cat E:/big.txt | wc       cat E:/hello-link
  cd E:/projects       (a subvolume: a separate tree of its own)
  cp E:/hello.txt C:/  echo hi > E:/new.txt  (fails: read-only)