Differences between RAID100 and RAID 0 arrays?

RAID 100 as stated in Wikipedia is a nested RAID level. It is said that RAID100 is a RAID0 built on RAID10 arrays. From this it follows that on-disk contents of RAID 100 will be exactly the same as in RAID 10 array.

Thus, it is possible to get RAID0 from RAID100 by removing one of the disks of each mirror pair. As fa as recovery aspects is concerned, RAID100 recovery is the same as RAID10 recovery which in turn just boils down to RAID0 recovery.

Unformat and undelete - what's the difference?

There are two widespread types of data recovery - unformat and undelete.

"Undelete" usually describes a recovery of one file, or a few files from an properly functioning filesystem. What to expect from the undelete process in various filesystems is explained in detail at www.un-delete.com.
The term "unformat" usually means a recovery of multiple files off the damaged volume on which none of the files are accessible any longer.

The "undelete" software assumes that the filesystem is OK, all but one sought-for file. Conversely, for the "unformat", the data recovery software must expect to properly work around some massive glitches of the partition, for even the quick format overwrites parts of the filesystem with blank data.

If the files are recovered corrupt after unformat, it is in most cases pointless to apply a file data recovery software. The "undelete" is in most cases very quick and produces cleaner recovered files.