If the memory stick was formatted as NTFS or FAT/VFAT, there is a system process (I forget which one) that tends to end up keeping the device open and preventing you from in mounting the device. I’ve seen this a few times.
Attempting to unmount the disk while the process is still running, results in a "Device is busy" error message. The only way to unmount is to kill the process first.
For the life of me, I can’t remember which process it was. But to find out, I remember using
lsof /dev/sdx, where
/dev/sdx was the path to the device that’s being blocked by the process.
That yielded the name of the process that was keeping the device open. Then it was just a case of killing the running process and then it was possible to actually unmount the device.