Rsync error code 11 indicates that the Disk Space is Full
or the system is unable to detect the external drive
"some cheap 512GB flash drives online" sounds like they may be fake 512GB flash drives. Fake flash drives and SSD's are quite common.
Back when 16GB flash drives were expensive, I bought a Kinston Traveler from a hawker outside a New Deli computer store. The packaging was perfect and the drive looked perfect. I took it back to the hotel room and discovered that it was hollow inside. I take it with me when I travel.
A few years later I got a great deal on a 128GB flash drive from Amazon, it checked out okay on Gnome Disks but it only lasted one day.
Saw an ad on Amazon last week for a 16TB SSD's for $100. That ended up just being a USB external enclosure with a 64GB Micro SD card inside.
f3 is recommended for checking for fake flash drives: https://manpages.ubuntu.com/manpages/bionic/man1/f3read.1.html
If you get the flash drives working see also: https://askubuntu.com/a/1403793/43926
If you do not get it working, it might be good to carry on your keychain, like I do, as a reminder about deals that are too good to be true...
I like running Ubuntu from flash drives. Ubuntu will run in RAM if you have enough of it. RAM is much faster than SSD except when reading or writing to disk.