Microsoft notified its users that the May Windows 10 update won't install on PCs using USB storage or SD cards. The company says it's blocking the installation on those PCs because "inappropriate drive reassignment" might occur and could impact both external devices and internal hard drives. As a workaround, users will need to remove any USB thumb drives, USB-based external hard drives or SD cards and restart the update.
Via: ZDNet
Source: Windows Support