A few questions
Is it formatted NTFS?
Why do you want that cluster size (I use 4k on my SSD's)?
What OS are you running on the machine the disk is in (so I can suggest tools if you need to check anything specifically)?
Here's an example of one of my 60GB SSD drives
62417919 KB total disk space.
45361824 KB in 138463 files.
74300 KB in 139904 indexes.
0 KB in bad sectors.
518971 KB in use by the system.
65536 KB occupied by the log file.
16462824 KB available on disk.
4096 bytes in each allocation unit.
15604479 total allocation units on disk.
4115706 allocation units available on disk.
You'll notice 4k cluster size and about 700MB in indexes and system files (for NTFS not Windows)
One thing you can check easily is to open the disk in Windows Explorer and since it sounds like you don't have any hidden or system files on it (except for small ones that windows may make) you can select all files and folders on the disk, right click and choose properties and look at the difference between
Size On Disk:
to see the effect of your cluster size and if that is where the missing space is.