I've been concerned for a while about my IP cameras hammering the drives on my Synology NAS with non-stop recording, so I decided to find a way to ease the load a little bit.

An obvious choice would be to use one the USB (or eSATA) ports and plug an external drive to it. Unfortunately, Synology (Surveillance Station) doesn't officially support this - according to the forums the reason is that they couldn't guarantee the marketed performance levels.

Since I don't care about performance that much and I had a terabyte USB drive laying around, I found a way to overcome this limitation.

Great deal of inspiration was this thread on Synology forums: https://community.synology.com/enu/forum/3/post/132261

So this is how I did it:

1. Formatted the drive on my Windows PC. NTFS worked fine.
2. Plugged the drive into one of the USB ports on the NAS.
3. The drive was detected in the DSM admin portal and mounted as shared folder. This folder cannot be configured for camera recordings.
4. I created another, regular, shared folder - called it surveillance_usb.
5. Went to Surveillance Station -> Recordings -> Archive Settings. Then Add Storage -> Use an existing folder -> surveillance_usb.
   1. 
6. SSH'd to the NAS.
7. Found that my USB disk has been mounted as /volumeUSB1/usbshare1-2.
8. Back in the DSM web GUI I created a Triggered task, changed the user to root and event to Boot-up.
9. The user-defined script for this task is:
   1. mount -o bind /volumeUSB1/usbshare1-2 /volume1/surveillance_usb
10. I clicked Run to trigger this task manually.
11. In Surveillance Station I went to my IP cameras and changed Recording -> Recording storage to surveillance_usb.
12. After a short while, recordings started to appear on the external disk. Older recordings were migrated there too.

And now the NAS is storing recordings to the external drive. This method should also cover reboots, but I'm yet to see if it really worked :D