Do NOT partition a drive with data unless you use something like Partition Manager for the job. A Partition operation as Windows does it through Disk Manager (or during the installation of Windows) completely over-writes the existing Partition Table, leaving no trace of the old formatting structures (directories and allocation maps, etc), so you lose everything. After that you must run a Format operation to get the disk ready to accept data.
Partition Magic and some similar tools do not work this way. They will read and revise an existing Partition Table, advising what you can and cannot do, and fix up the directory etc. They even will move existing data to the right places on the disk so it is all in your existing formatted partition, before they co ahead and create and Format a new empty partition. Windows won't even try this - it will simply blunder on and assume your drive is empty and you don't want any data on it.
IMPORTANT!! Even the best software like Partition Magic can have problems. Like, a power surge comes along in the middle of their work, or the dog snags your power cord and pulls it out of the wall. ALWAYS make and test a backup of all your stuff before doing major surgery like re-partitioning.