Another factor is if this is your only drive or is a scratch-disk for Photoshop or other memory-intensive software. Photoshop wants to grab a chunk of your disk drive to use to spool the pages and history states as you work on your image. You may need to change the scratch disk hierarch in your PS preferences settings.
Otherwise, the rule of thumb I was told years ago is to keep at least ten percent of your hard drive free just to keep the thing from having issues when you use it.