Refer to page 100 of the 880ex owners manual - pdf version - downloadable from rolandus.com, if you don't have the hard copy.
This describes the drive initialize process. This process wipes the entire drive (regardless of whether or not it is partitioned). The 880ex does not contain a defrag operation, so this is what we get - wipe the drive, or not.
If you don't want to go that drastic at this time, you might be able to recover a significant enough amount of space for a bit more recording by using SONG OPTIMIZE on each song file you currently have on the drive. Song Optimize will delete any references to recorded data you might have erased, recorded over, or otherwise deleted from a song, if you didn't use UNDO to get rid of it already. When you do Song Optimize, it will free up whatever space it can and you will no longer have Undo levels to recover old data from. You probably won't recover enough data to extend your available free space by very much and this is certainly not a replacement for the need to back up your Songs and wipe the drive now and then.
If you have significant Songs on the drive that you want to save, then you need to first find a way to back up your data, if you don't already have a method, before you do anything as drastic as to reinitialize the drive.
You should also learn to understand how a drive is initialized and partitioned in the VS recorder. One hint about initializing a drive - don't turn on surface scan unless you have a significant reason to search for bad sectors and a lot of time to waste while it does its thing. Surface scan is rarely needed, I think.