I just got this pop-up:
Ar? 
How would my startup disc become full, and what should be deleted?
It happens. Even if you don’t think you’ve downloaded or installed too much to your drive, it has a way of filling up. Sometimes with behind-the-scenes files that get bloated over time.
First, download Cocktail (free):
It’s a great general-purpose utility program. First, use it it (in the Files tab - Localizations) to delete all the unnecessary localization (language) files. If you only ever need English, you can save about 8gigs or so of drive space just by doing this.
Then in the same tab but in Caches, clear your caches. This can save a lot of space as well.
If you still need help, download Disk Inventory X (also free):
Have it scan your drive, and it’ll show you, in a nice, easy to understand graphical format, what files are taking up so much safe, and help you delete them easily.
On further investigation, I’ve found that the startup disc should clear after a reboot – which I only do if I get locked up, so it’s a rare occurrence. I’ll try that later. (I’m working right now, and I want to finish what I’m doing first.)
I also found MacJanitor:
I leave my computer on, but I put it to sleep before I go to bed.
It’s very, very unlikely to be anything that MacJanitor is going to fix.
Run Disk Inventory (or Grand Perspective), and see what enormous file you have forgotten about.
Any harm in installing MacJanitor anyway?
Well, it’ll take up some disk space, which you seem to be short on.
I second the recommendation of Disk Inventory. Have you emptied the trash lately? The space isn’t actually cleared until you do so.
Trash! I always forget to empty that. (Heck, I have to keep the trash/recycling schedule on the fridge so I don’t forget the bi-weekly pickup.)
That’s done now.
There’s also OmniDiskSweeper, similar to Disk Inventory X:
I second beowulff, MacJanitor probably won’t help (and looks quite old).
I had an issue with way less than 1 GB on a 160 GB HD. I found that all the photos I deleted in iPhoto were actually still in iPhoto trash. Not flushed with the regular trash. When in iPhoto and I moved to trash and deleted, I freed up about 20 GB.
How full is your startup disk? Generally, at least in my experience, Macs run best if you have at least 10GB of free space on your startup disk.
I don’t know how to check.
Click on your boot drive and hit Command-I. What numbers do you see for “Capacity” and “Available”?
You mean ‘Macintosh HD’? 2.79 gb.
It’s whatever you called it. But, yeah, Macintosh HD would be the default.
2.79 gb free is pretty skimpy, especially if you just emptied the trash and you have nothing else waiting to be deleted. (What’s the total capacity of the drive?) You need to free up that space up. Delete files you don’t need. Clear your caches. If you have other hard drives with space on them, move stuff off your boot drive to those drives. The programs listed above should help you see where all the clutter is.
It’s 93 gb.
I assume that the capacity is somewhere north of 100 GB?
Podcasts are another thing that can eat up hard drive space if you’re not careful (I had this problem earlier this year). This is doubly true if you use iTunes and tell the computer to “Keep File” rather than “Move to Trash” when you try to delete them. By default, podcasts are kept in Music > iTunes > iTunes Music > Podcasts, so if you’re into podcasting you might see if that needs to be cleaned out.
Yeah, they say you should leave around 10-15% free space on your drive, but I’m not convinced that the actual %age matters so much as the actual amount of disk space. The Mac needs disk space for its swap files and things of that nature. In my experience, if you’re running lightweight apps, you can pretty much run your startup disk down to the last gig, but stuff like Photoshop and the like, they don’t run well at all with low startup disk space.
I don’t have Photoshop. About the ‘heaviest’ programs I have are Word and Excel. I use this machine for browsing, and for connecting to work with RDC.
Still, personally, I’d clean it up to have around 10 GB free, but if you’re not noticing any performance issues, you may be fine.
I downloaded Disk Inventory X, and found some movies I’d forgotten about. I deleted four of them, and now I have 5.87 gb free.
I can clear more space by deleting emails, but that’s a bit of a chore. (Maybe just the ones with attachments.)