"Inappropriate drive reassignment can occur on eligible computers that have an external USB device or SD memory card attached during the installation of the May 2019 update," writes Microsoft. "For this reason, these computers are currently blocked from receiving the May 2019 Update."
For example, an attached SD card that previously had been assigned E: might be shifted to F: after the May 2019 Update is installed. This could cause issues if you have applications that are directly running from (or need access to) a SD card or USB drive.
What makes this external storage especially tricky is that it doesn’t only affect the drive letter assignments of SD and USB devices, but also your internal hard drives and SSDs. That would be potentially disastrous, so it’s understandable that Microsoft is blocking the update on these particular configurations.
However, the workaround for this issue is quite simple: just remove the offending SD card or USB storage device from your machine before attempting to upgrade to the Windows 10 May 2019 Update.
Microsoft does state, however, that the issue will be resolved in a future Windows 10 "servicing update" and that it has already been addressed in Insider builds 18877 and later.