mount: device or resource busy after enabling multipath

We have a heartbeat cluster with two nodes. It has been running for several months without problems. The shared storage is on an IBM DS3400, on which we have a large volume formatted with ext4.

Today I decided to reboot the active node for security reasons. So I’ve switched to the passive node, which failed at the first step: it was unable to mount the storage (/dev/sda1). After whining for a few moments, I tried to mount it by hand, which told me

/dev/sda1 already mounted or /data is busy

I’ve quickly made sure that none of that was true. After checking this-and-that, it turned out that the passive node had multipathd running, so I looked under /dev/mapper, and found two symlinks there, <long-long WWN> and <long-long WWN>-part1. As the partition table and the disk size was the same as on /dev/sda, I tried to

mount /dev/<long-long WWN>-part1 /data

and voilà! It worked like charm!