Try this with an Ubuntu LiveCD (here) or GPartEd boot CD (here) or something similar where you can boot from CD and not your hard drive partition (assuming you have another computer where you can download and burn one of those). Even if they don't understand the HFS+ file system they will be able to read it in raw format.
This will be the safest way. For once, the operating system will not lock the drive and you won't run into the risk tha the swap file or temp files will overwrite the precious free blocks.
First off, the pdisk command is used to modify drives that are partitioned using an Apple Partition Map. Only PowerPC-based Macs use those.
Secondly, your umount commands above are failing because /dev/disk1 and /dev/disk2 are not mounted. Typically only partitions are mounted (e.g. /dev/disk0s3), although in your case, you have a Fusion Drive. You are absolutely correct that /dev/disk3 is not a 'real' disk. It is the combined capacity of /dev/disk1s2 and /dev/disk2s2. If you type umount /dev/disk3, that should work. Of course, nothing is mounted on /dev/disk1 or /dev/disk2, so you will have to issue a diskutil unmountdisk /dev/diskN for those. That should free them up so the gpt command will stop giving you the resource busy error.
Fusion Drives use Apple CoreStorage partitions as containers, which is sort of analogous to Microsoft's Dynamic Disks. Unfortunately, they make manipulating the partitions more difficult, and take many disk/partition recovery products off the table.
However, aside from the issues above, I'm not exactly sure what the problem is. You mention that the partition table is lost, but the output from diskutil list suggests your partition tables seem to be OK.
Can you elaborate a little more about the problem you were having?
Best Answer
You cannot override this behaviour.
The operating system states that it is busy. The operating system is probably correct. The error message could be for these reasons:
As you wish to delete the file, my suggestion has been to boot the machine from a linux image, mount your file system and delete the file.