When you talk about Data migration. First you need to know the type of Data, like DB, CIFS non structured data, etc, what kind of file system you migrate, from what technology to what technology. Is the File systems (FS) are SAN, NAS, directe attached, Virtual Luns,
After you need to know who is accesing the data, thoes the data is sencitive (you will need to know what toes mean sencitivity for your organisation)
You also need to know where is the data is going, Tier 1 disk (mean SCSI, Fiber SCSI High hand Disk) Thier 2 disk, SATA, Fiber Sata (FSATA),
The migration risk to impact the backups, so make sure you have enough backup space.
and you need to consider the tool you will use to do the migration, script, SAN/SAN manufacturer tool, Software manufacturer tool like Quest.
and you will need to plan a Lab to do test, a pilote this is end users that test live data after a migration and at the end what is the migration plan how many user at a time, time frame for the migration.
Their is some of the thing you should consider for your data migration
P.S. Sory for my english my first language is French.