Our mirror at USTC is down (in fact, for quite some time now) due to hardware failures in their hard disk arrays.

That said, you might need to change your APT source lists to install and update packages normally. Use apt-gen-list -l to see your options (avoid 10-ustc for now), and apt-gen-list -e "repo1 repo2 repo3" to enable new mirrors to use.

We will post another notice when USTC's mirror has resolved this issue.

Relevant documentation:

• USTC's notice about their hard disk array failure.
• USTC's update to their previous notice, they have decided to perform an offline maintenance.