I didn't have any USB devices connected, and i had disconnected all my HDD/SSD/Optical drives except for the C:\ drive,
and Windows 10 would still fail to update to 1903 on my X58 computer.
After a few weeks of having this problem, Windows update would tell me my device wasn't ready for 1903.
That was until today, when the update was offered through Windows Update once again, without said message.
It updated just fine even with all my drives connected, and took me straight to version 18362.239.
It seems Microsoft has finally fixed the issue of older devices not being able to update to 1903.