Backup onto the to-be-backed-up-drive itself...
 

During the course of making a backup to an external USB drive using SD (v. 2.5) the drive was erroneously ejected (not giving any warning) and disconnected. Neither did SD give a warning and continued its activity until a few minutes later that I disocvered the problem. I stopped SD immediately (which gave the usual warning about instability), but in the few minutes SD had continued copying files from the boot disk onto itself.

I do have 4 disks in my MP, one of which is the boot drive, two are configured as a RAID0. Disconnecting any external drive and running Terminal I found this:

MYMAC:~ admin$ sudo du -hk -d 1 /Volumes
Password:
17498556 /Volumes/BACKUPCLONE [here should be my boot volume's name "OSDRIVE"]
72776884 /Volumes/BOOTCAMP [that's a partition on my bootdrive "OSDRIVE"]
385343780 /Volumes/THIRDDRIVE
1280885480 /Volumes/USERSDRIVE [that's a RAID0]
1756504704 /Volumes

"BACKUPCLONE" is the name of the external backup drive and should not appear in this list. Instead there should be my boot drive "OSDRIVE" with a much bigger size. DiskUtility shows the correct volume name.

I've had this problem already a few years ago and solved it by a complete restore. Since there must be a simpler solution to this problem, I am urgently asking for help: How can I set the boot volume right again?

Thanks!

"BACKUPCLONE" is a folder at this point, rather than a mount point. You should be able to delete it to recover the space.