Leia aqui no quarto parágrafo.
Nearly all USB flash drives pretend to have a removable media (even it's a lie), so Windows detects them as 'removable'. On drives with a removable media Windows 2000 and higher supports only one partition.
Finally it's a single bit in the device's device descriptor, the removable media bit (RMB). If you take away the RMB then Windows sees the drive as 'local disk' and multiple partitions work.
The Removable Media Bit can be changed either in the drive's hardware or by a Windows filter driver.
Changing in the hardware works with some flash drives by means of the tool Lexar BootIt.