mysqldump (in crontab) failing with 'errno 32' even though it worked last week

I use the mysqldump command (as below) in crontab to do a dump of my database each night.  Up until last week this was working fine.  Last week (when I was out of the office) there was a problem with the db and a colleague of mine had to do a manual dump.  When he tried to run mysqldump (using the same command as I do) he got 'errno 32 on write'.  
I just tried to run the command myself and also got 'errno 32 on write'. My cronjob has also started failing even though crontab has not been changed.  I am very confused given that this exact command was working up until last week.  Can anyone shed light on what is going on?  This is the command I use, run as super user:
/usr/bin/mysqldump --all-databases -u root | /usr/bin/gzip -c > /backups/mysql_backup.gz