It seems like unraid compares the number before the K, M, G, T instead of the actuall calculated size.
Example 1: change capacity from 500GB to 1TB
1. I first set one vDisk to 500G
2. Wriiting 1T is not accepted by unRaid
3. Writing 1024G is accepted, and converts 1024G to 1T
As far as I know, decreasing capacity is not supported by unRaid...but this bug enables it
Example 2: decrease capacity from 900GB to 500GB
1. I first set one vDisk to 900G
2. Tried to set capacity to 500G, but that does not work
3. Set capacity to 1024G, which i converted to 1T
4. Set capacity to 500G, works just fine
Also. If you want people to not reduce the size of a vDisk, you should probably not have the calculation visible in the url-field. In Example 3, I could just change the url from "...&oldcap=900G&cap=500G" to "...&oldcap=10G&cap=500G" to bypass the entire system.