Microsoft has released the Windows 10 version 1903 KB4497935 cumulative update to Insiders in the Slow and Release rings for testing before being pushed out to everyone. The update has a long list of fixes, but the one that stands out the most is a fix for an issue preventing some users from upgrading Windows 10. In April, Microsoft issued a support article stating that Windows 10 upgrades would be blocked if a USB drive or SD card was plugged into the computer. This was because the drive letters for these devices may be reassigned during the upgrade, which could cause issues with the operating system and software.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-1903-insiders-test-fix-for-usb-drives-blocking-upgrades/