There are plenty of references to do this on the internet such as
this but the idea is as follows:
1 - Format the drive
2 - mount it into a folder in ClearOS
3 - modify fstab to make the mounting permanent
You have a few ways you have a Windows share there:
1 - Configure samba manually to create shares there
2 - mount the disk into the /var/flexshare/shares folder and use the flexshare as normal
3 - define a
custom flexshare directory
4 - look at the mount command with the bind switch to mount the drive into an existing flexshare folder/sub-folder
If you use method 2 (possibly the easiest), move any data you have in the flexshare to your new drive first or you will not be able to see or access it. (You won't have lost it, it just becomes invisible)
[edit]
Marcel beat me.
[/edit]