Microsoft released a support document (opens in new tab) that addresses an error that may pop up in the future update, stating that "This PC can't be upgraded to Windows 10," and the apparent cause is that the computer simply has "a USB device or SD card attached."
To be clear, this error message is deliberate, as Microsoft stated that "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." This isn't the first wonky thing to happen to Windows 10 updates, but it still seems quite annoying.
Microsoft's "workaround" is to unplug any USB storage device or SD card from the system and reboot it. If, for some reason, that you're running Windows 10 from external hard drive or thumb drive, then you won't be able to update it.
Interestingly enough, this error actually won't pop up if you're on a build prior to the April 2018 Update (Windows 10, version 1803) or the October 2018 Update (Windows 10, version 1809), so if you haven't updated in awhile, it actually benefits you in this case.
On top of all that, there's a little note at the bottom of the document that states "The drive reassignment is not limited to removable drives. Internal hard drives can also be affected." Which, by definition, probably means that some may be permanently blocked from the Windows 10 May 2019 update.
Apparently, this issue is already resolved for Windows Insiders in build 18877 and later builds as well. However, it could lead to a rocky public launch for the May 2019 Update. If you want to jump into the updated version, you can join the Windows Insiders program in order to avoid this issue.