Window's disk management can be absurdly stupid at times. If you proceed it'll likely show an error anyway. When I had the same problem I just resorted to an alternative software.
The software I used? Forgot! However I would recommend grabbing GParted, writing it to a USB, booting off it, and doing your re partitioning there.
If you instead want to use what's on your USB already (I'm assuming a Linux distribution of some kind) you can simply boot off that, and use it's disk utility to resize partitions for your needs. Distributions like Ubuntu have these functions built into the installer.