Question: Is it safe to de-fragment your disk when you have low disk space? What could be the consequences? What are the advantages and disadvantages of de-fragmenting. Is it possible to resume de-fragmentation of a disk after a few days? Can de-fragmentation of large disks be simplified as it takes lot of time to complete? Is it possible to lose data after de-fragmentation? Will it reclaim my Disk-space?

Answer: 
" Is it Safe to De-fragment your Disk when you have low disk space, what could be the consequences?"

The defragmenter will not complete the defrag efficiently if you have low free space if you use the Windows defragger. But you can run a third party automatic defragger which defrags under less than 5% free space and get the job done without any problems.

"What are the Advantages and Disadvantages of De-fragmenting.Is it Possible to resume de-fragmentation of a disk after a few days. can De-fragmentation of large disks be simplified as it takes lot of time to complete. Is it possible to lose data after de-fragmentation. Will it reclaim my Disk-space."

There are no disadvantages, advantages are that the performance will be smooth and data access fast.
With intelligent automatic defrag technology it's also possible to have a program defrag in the background without being resource intensive and will also defrag very fast as compared to the built-in program. Defrag doesn't create any new diskspace and there is no risk of data loss. Just ensure that you have a power backup for the PC.
There is no risc for you. But the less space is available, the longer will the defragmentation need.

Defragmentation means: re-arrange the data on disk, so that (if possible) all the data in one file sits togetcher, physically on the disk. The if there will be a gain in disk space it will be very, very few bits.. 

Defragmentation can take a long time, depending on how large the amount of data /disk is and depending on the fragmentation found on the drive.
You can stop and restart the defragmentation at any time in most defrag-tools.

Here a visualisation.. Imagine every char in the following ascii chart is a sector on the disk and a

* "." means: free space
* "+" means: file A
* "\*" means: file B

Here's your disk:

```
.+.++*..**+.*

```

The system will need to read at several addresses to get the content of file A!
After defrag your disk looks like this:

```
++++..****...

```

Your system can now read file A or B without readjusting the physical needle on your disk.. so this will be faster.