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: 
The most annoying consequence will be that defragmenting (since there will be files moved around) will be very slow and take a horrable amount of time. Advantages are, that your files are not fragmented on the harddisc and this will lead to a speedup unfortunatly with very low disk space the fragmentation effects increase dramaticly so that your hard disk will get fragmented again very fast and all defragmenting will be for nothing. 
There are some commercial programs who promise better results, but i can't tell you if they are any good. 
And to the last question: it will not recraim your disk space. The amount of free space will stay the same.
As i understand..... Defrag has no risk.Yes it takes hell a lot of time.while you save a file datas are written here and there over the spinning media where it finds empty space.so when you read that particular file data has to retrieved from so many places as it was not written in sequence which makes the program consume more time. By de fragmenting system pick up the fragments and arrange in an order which makes the systems read files faster.Thats it.