Windows Upgrade Help and more

Thanks! I copied everything over, but now my external drive is saying it's full and I'm not sure if it got all my photos.

I'm trying to upgrade my Windows 10 PC to Windows 11, but I'm really nervous about my files. The upgrade assistant says my drive is compatible, but I keep getting error code 0xC1900101 right before it starts. My main worry is losing years of family photos and documents. What's the absolute safest ...