Partition still 50% full despite deleting all files

PowerEdge 2900

Disks as per factory setup were:
2 x 136GB in RAID1 configuration

Partitions on virtual disk as per factory set up:
C:/ (98.07GB)
D:/ (16.53GB)
E:/ (21.48GB)

Moved Exchange logs and databases from C:/ onto E:/ for space reasons a while back. Have now installed new Disks and moved Exchange to these and deleted all files and folders from E:/. All is working fine.

However E:/ still reported as having only 10.29GB free space (48%). I.e. 11GB is 'in use'. I have turned on 'show hidden files and folders', but nothing shows up. I'm aware that some small amount of space would still be used... but 52% seems a bit excessive.

Any thoughts on how I can 'see' what is there and how to recover maximum amount of space back from this partition.

Thank you