https://arstechnica.com/gadgets/2019/04/windows-10-may-2019-update-blocked-for-anyone-using-usb-or-sd-storage/

While it's not officially out yet, the Windows 10 May 2019 update is available to Windows Insiders on the Release Preview distribution channel (and also to MSDN subscribers). So anyone who wants to get a head start on the next major iteration of Windows 10 can do so right now—unless they have USB storage connected to their PC.

Because of an issue that's frankly remarkable, Microsoft is blocking the update for anyone using USB storage or SD storage. That is to say: if you have a USB hard disk or thumb drive, or an SD card in an SD card reader, the update won't install. Perhaps more strangely, this is only the case if you're currently running version 1803 or 1809; upgrading from 1709 or 1703 (both of which are still supported, at least for Enterprise and Education users) means everything is, apparently, fine.