is to replace the internal hard drive and install OS X on it.
Then get a USB enclosure and install the full hard drive inside it.
Once the MBP is running correctly off the new internal hard drive, connect the USB enclosure to it and drag back whatever files it is that were, apparently, too important to get rid of.
You do, of course, have a full backup of your MBP?
My MacbookPro running OS X, just ran out of disk space. Strangely even when I moved out over 70GB of data, the available space still showed close to zero. I emptied out Trash too. All available space keeps getting filled up instantly. Tried to erase free space using Disk Utility (maybe this was a mistake). After that, the free space became an absolute zero and now the Mac does not boot. Stops at a blue screen with a Settings (gear) icon and HD icon at the right top, not responding to any mouse inputs. Only option at the point is to restart by pressing Power button.
Any help please. Have an important deadline to meet and this couldnt happen at a worse time.