Checking disk usage
You can check disk usage in 4 places, welcome page, ruTorrent, info page, shell.
Master GUI places
ruTorrent bottom left shows a percentage used of your HDD Quota. Most browsers support moving your mouse over the percentage and showing an caption of exact numbers plus burst limit.
Info page shows detailed quota information, the exact details of used, limit and burst as reported by the system.
On the welcome page you see this value on right top, in the rightmost bar.
It's empty, what now?
You are probably over quota, you can verify this on shell. This bug is to be fixed on future PMSS release.
You can use du and quota to see your disk usage in shell.
cd ~ du -hsc
will show total used and quota -s will show quota information in human readable format.