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.

RAW file system problem

It may happen when you take out the storage device (memory card) not using "safely remove" feature, the next time you connect the storage device you can receive a message "You need to format the disk in drive X: before you can use it".

Typically this means that the filesystem on the drive became RAW. Obviously, initial file system was corrupted because the data stored in the buffer during disconnection of the drive was lost due to inappropriate disconnection.

To get rid of the RAW file system issue you need just to format the drive, however, be aware that your data would be gone forever according to the format type you apply. If the data is important you need to recover data first and after that do the format procedure. To recover data you can just use some data recovery program.

Booting from a SW RAID

In the previous post I said that you cannot boot from the software RAID array. In fact, this is not true. All the widespread bootloaders, from Windows NT, Linux LILO, and GRUB, would successfully load OS from a RAID 1 (mirror volume).
It should be noted that some further actions must be undertaken for RAID 1 boot volume when you plan to build a RAID.

Since the mirroring technique does not extend to the Master Boot Record (MBR), you must copy the MBR manually between drives in a software mirror. Otherwise, once the primary (boot) drive fails, you will end up with an unbootable system.

Surely, there will be no irreversible data loss - all you need is to just mount the disk into some other machine and read it, but you get no automatic recovery in case the disk fails in operation, requiring a reboot.

Make a software RAID bootable?

Is it possible to create a bootable software RAID0, RAID 5, or spanned volume containing a Windows installation?
Unfortunately, no. It is not possible to boot from a software RAID0, RAID5, or spanned volume. A hardware RAID controller is required to do this.

You cannot start an OS from the software RAID because the RAID is not accessible until the OS is fully started up, and the operating system itself is on the array.

Why it is critical to read data off both disks in RAID1?

When using RAID 1 it is needed to access data stored on the second member disk as well. Otherwise it may lead to that a bad sector appears on the second member disk. In this situation the RAID 1 doesn't have redundancy any longer and on top of that you do not know it. Absolutely the same is true for hot spare drives.

Typically one of the causes of this issue is the array being idle. To avoid this situation you can inspect both copies of data while reading. It should be noted that heavily loaded RAIDs do not have such issue.

In Windows, at least up to Windows 2003, RAID1 software implementation usually reads from the first drive, rendering RAID 1 unreliable.

NAS and backup power

Just cutting off the power is not sufficient to test an UPS. Generally, Network Attached Storage receives the messages from its UPS unit constantly and reacts at once. It should be noted that during the start up the notifications about power problems are not received. Additionally, one should take into account that the entire cycle (start up then shut down) may require several minutes.
 Consider the following situation:

1. power failure happens
2. NAS takes power from the UPS battery for some time and then shuts down
3. the grid power turns on and the Network Attached Storage powers up and begins to start up
4. here power is off again.

Now NAS cannot process a report about power failure event until it properly starts up. Once NAS loads, it finds out that the power failure has happened and commands to shut down. Further it may happen that UPS is not able to provide power long enough (to shut down NAS correctly) for the battery is already exhausted during the first cycle.
 One can avoid this by setting the NAS up so that it shuts down as soon as it gets a message that power failure happens.

 All this allows saving battery charge to do one more cycle.

 Alternatively you can try to set Network Attached Storage up in such a way that it does not power up automatically, but per operator's command after the power failure has happened.