To run a programm that doesn't quit if you close the ssh session use nohup (no hangup). Attention, you have to run it as root for it to work!
admin@DiskStation:~$ sudo su Password: ash-4.3# nohup <command> & maybe: ash-4.3# nohup cat /dev/zero | split -b 4095m - /volumeUSB2/usbshare/zeros -d --additional-suffix=.file &