EaseUs has one of the most sophisticated algorithms for conversions from FAT to NTFS, or Dynamic to Basic without data loss, and they clearly state:
2. Before you convert a disk, be sure to backup the disk that you want to convert to GPT disk. Converting the disk requires that all partitions and volumes are deleted on the disk first.
vishnuip - can You please post here model of Your motherboard? Do You use newest available BiOS? Do You run HDD in AHCi mode, or in iDE/Compatiblity mode?