Once you can boot to free disk space here are hints on how you can do that.
Restart in Safe Mode:
http://support.apple.com/kb/HT1564?viewlocale=en_US
Empty the trash.
- Delete all files in the Downloads folder.
Empty the Trash.
- Start iPhoto. Empty its trash.
- Delete "Recovered Messages", if any.
Hold the option key down and click "Go" menu in the Finder menu bar.
Select "Library" from the drop down menu.
Library > Mail > V2 > Mailboxes
Delete "Recovered Messages", if any.
- Empty the Trash.
- Restart.
- Re-index your system disk:
http://support.apple.com/kb/ht2409
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Backup:
Run a Time Machine (or other) backup since you are about to delete and move files and you may need to recover from any inadvertent mistakes or decisions.
Deleting and Offloading files:
Use OmniDiskSweeper find large folders and files to delete or offload. If you are running 10.8 or 10.9 download it from http://www.omnigroup.com/more. If you are running 10.6.8 or 10.7 down it from https://www.macupdate.com/app/mac/7402/omnidisksweeper. Do not touch any system files. Do not offload files onto a Time Machine disk. Do not offload your entire user folder. Do not rely on restoring files from Time Machine; delete only those files you never want to see again. Do delete or offload seldom use folders/files in your user folder. Remember to empty the trash after trashing the files.
To have Time Machine backup your system disk and your external offload disk up onto your Time Machine disk:
> System Preferences > Time Machine > Options… > Remove the offload HD name from the exclusions list.
From: "More ways to save space if you have a spare partition or second hard drive."
http://www.thexlab.com/faqs/freeingspace.html
- Compress and archive old files.
- Clean up system logs and temporary files.
- Uninstall Mac OS 9.
- Move your iTunes Music folder to another disk or partition.
- Move your iPhoto Library folder to another disk or partition.
(Includes instructions on moving subsets of the libraries.)
Additional references on freeing disk space:
http://pondini.org/OSX/DiskSpace.html
http://www.macmaps.com/diskfull.html