The error can be ignored. This is due to the fact that the virtual CPU thinks it has missed some time, i.e. has been locked up, while in fact it was suspended and couldn't get scheduled while unsuspending.
As for the load, it's normal, reading a few GB of data into RAM, mapping the RAM to the VM's page offset takes loads of CPU and disk operations.
As I was expecting a quick answer which didn't came, I have subjected an older laptop to a rough test intended to provide the answer:
I have opened a text document, written something and, without saving the document, entered hybrid sleep with the command systemctl hybrid-sleep. At this point the LED beside the power button was blinking, like in Sleep mode. As the laptop cable was disconnected, I have also removed the battery. At this point the machine was fully shut down, the LED was dead.
Putting the battery back and starting the machine from the power button, my multi-boot list became available (Windows etc.), and when booting Linux it all went just like in the case of hibernation (message with booting from dev/disk/...), my open unsaved document was available.
So, the answer is YES.
Hybrid Sleep is not an intermediary state between Suspension/backup to RAM ("Sleep") and that to disk ("Hibernation"), but a double operation; it simply does both.
It ensures all that is needed for the Hibernation procedure to work, but it doesn't shut down the machine; instead, it suspends it to RAM (Sleep mode). As long as power is not completely drained, only the 'Sleep/Suspend to RAM' capability is visible. In case of total power drain, the Hibernation capability is used: the machine is shut down but is able to restore all programs from swap upon startup.
It is important to know that in the case of Hibernation/Hybrid-Sleep, the boot/startup procedure should not be interrupted (by the power button or power failure) or the programs' session&data saved on swap-partition (by the 'hibernation' action) will be lost.
Best Answer
since you only use the secondary hdd for backup, I would suggest telling fstab not to mount the drive automatically and to make a backup script that mounts the drive, makes the backup and unmounts the drive again.
example of the fstab line with the noauto option:
bash script for the backup would than begin with
mount /media/backupand end withumount /media/backup