Independent disks are not being backed up by design, but they are included when calculating the needed backup space.
I have vm with an independent disk that's meant to be excluded from backup. That extra disk is about 7TB.
I use --backup-room=400.
What is happening, is that first, the needed space is calculated (which is over 7TB in my case), then xsibackup attempts to free 7TB but is constrained by the --backup-room=400. Only when actually backuping it notices that the 7TB is actually an independent disk and does not need to be backed up. So everything goes as planned, but I do end with a report in my mailbox: "XSIBACKUP errors detected in backup job!!"
Seems to me that it would make more sense to first detect any independent disks, then do the space calculation.
As a workaround, I used the --backup-type=custom --backup-vms="myvm!myvm_1.vmdk" in order to exclude the disk manually. No errors this time, but I do get an email with "XSIBACKUP some warnings were raised". I'd prefer an email saying everything's fine, as that disk was excluded by design.