How ugly customer-hating synology is: Synology DSM os measures free space in percent. So when on your multi terabyte volume 200GB free remaining, you would receive notifications until it’s about 100GB free and then you will not receive notifications even it it runs out of space completely.

You can set the free disk space threshold percentage but it will accept number no lower than 5%. This is obviously a rudiment from old times, 20 years ago when volumes were 1-10GB, not TB.

I asked them to change threshold from percent to GB, over multiple times over few years - not going to.

I asked them to allow change threshold to lower value than 5% - not interested.

Not only you can’t know when disk drive runs out of space, but excessive notifications when there’s still lot of free space desensitises, so when actual disk failure happens you would ignore notification.