After a while, as I was warned by
@insanity, I did a try.
Consider that I've been using OBH 4.3 Full Backup without problems since it first release apart from the first attempt, when it was installed in OMB... but I thought it might have been a "youth" problem of the new release.
Then I installed OBH 4.3 in flash and it performs a scheduled full backup everyday, without issues, in my NAS. I also sometimes did some manual FullBackups to my HDD.
Today, after this warning from insanity, I tried to perform a FullBackup of another instance of 4.3 installed in OMB: it was a disaster: after having tried to build the backup (I had chosen the HDD, in this case) not only I got the same error lines in post#1 but, more importantly, it was impossible to restart also the image in flash, exactly as
@insanity had reported to me.
This means there must be an issue only if you try this operation in OBH running in OMB.