Here's what I did, in minute details to help those who feel uneasy :
- You will need an 8 GB USB flash drive, the OS X Mavericks installer file (you can redownload it from the App Store if you haven't kept a local copy), DiskMaker X. All this on another Mac you'll have to temporarily borrow to create a USB-based Mavericks Installer.
- Using DiskMaker X, create a USB-based Mavericks Installer on the 8 GB USB flash drive, following the simple instructions. Aside from having to tell DiskMaker X where is the USB flash drive you want to use and where is the OS X Mavericks installer file (if it isn't on the Mac's boot partition), you don't have anything else to do except to wait for your USB-based Mavericks Installer to be ready.
- Insert your USB flash drive (containing the Mavericks Installer) in a USB port of the problematic Mac.
- While keeping the option (alt) key depressed, restart your Mac and keep the key pressed until you see an array of possible startup disks on screen.
- Select the USB flash drive (containing the Mavericks Installer) in the list, and click on the arrow underneath to launch the Mac from the USB flash drive
- You will be greeted with the usual Recovery disk interface
- Select Disk Utility in the list
- Select the problematic drive (and not the partitions on that drive) in Disk Utility
- Select repair (for me, everything was repaired and was just fine at this point)
- Select repair again and repeat until no further repairs are necessary
- Check the partitions, to be sure.
- Quit Disk Utility
- In the Apple Menu, select Startup disk
- Select your usual startup disk on your Mac (the one that was broken) and restart
- When you hear the chime, you can remove the USB key
- To be sure, I restarted again in Recovery Mode (Command-R), but this time from the main drive, and everything was fine in Disk Utility
- Restart your Mac normally and be happy.
- Make a backup of all your file immediately afterwards, just in case this is an impending doom warning...
MacBook Pro, OS X Mavericks (10.9), 15" late 2008