|So you would rather gamble on losing the data?|
If the 1.5TB is only half full you could copy the data from the smaller drive to the larger and at least have the smaller one free to format.
If you make DVDR disks as the data is accumulated the task is manageable. Only you know what portion of the data on those drives is irreplaceable.
You do understand that FAT32 has a 4GB file size limitation. If you have that much data I would guess there may be many files on there larger than that.
If you are not inclined to burn files to disk then buy more external drives and format them to FAT32. Then copy the files from one to another. There is no problem in doing that, other than the 4GB file size limitation.
I also suggest that if you really want to protect that data then buy an external enclosure with space and a cooling fan then buy brand name drives to fit into the enclosure.
These cheap plastic enclosures that are barely larger than the hard drive are just a failure waiting to happen. Look at the links below.
If you decide to go this way then read the reviews on any product and be sure to match the interface types so the drives will fit and the enclosure will work on your computer.
eSATA connections are by far the fastest. USB 2.0 the slowest. Also make sure any enclosure you buy has an external power supply.