What is it? How do you fix it?
The best thing to do with all Cydia errors is Google them. I know, I know everyone says google it, but it works. Ya know just incase you have another error you can’t figure out:)
For this one it’s simple. You don’t have enough space to install the package your downloading. Hence the failed in buffer_write. The first thing you will say is I have X amount of space. Yes you do, but you also have two different partitions which things stay. Think of this like two hard drives, well the one that Cydia stuff installs to is full.
Reference the iPhoneWiki : LINK
You can use Cydia to check this space. On Cydia homescreen scroll down and you will see Storage Information. Click it to see your space.
To fix the error, you can uninstall things from Cydia, or the new hip thing is to install this thing called Stashing that moves files from one place to the other. While I don’t particularly like this method it’s very popular right now and is an easy fix. Go here to check it out. LINK