Should I defrag my 1.5 TB HDD BEFORE transfering files to a SSD? OR...

SSDs should never be defragmented, as doing so would not serve any useful purpose and would just accrue against the finite (although very large) number of writes an SSD can perform during its lifetime.

In fact the defragmenter that is built into Windows 10 will not defragment an SSD. It will perform a process it refers to as "optimization," which I believe corresponds to what is referred to as "TRIM."

Also, at best defragmenting the HDD prior to transferring the files will result in a slightly faster transfer, but the time required to perform the defragmentation itself stands a good chance of making the overall process take more time, rather than less time.

Regards,
-- Al

It doesn’t just defrag it optimizes files and their positions or location on the disk. If you do this before you load programs especially , it can be helpful. Your drive can access files in a more orderly fashion, which makes it faster.
Fragmentation happens when those files get split between blocks that are far away from each other. The hard drive then takes longer to read that file because the read head has to "visit" multiple spots on the platter. Defragmentation puts those blocks back in sequential order, so your drive head doesn't have to run around the entire platter to read a single file.

Do not defrag SSD drives. Your OS will likely preclude you from proceeding. There is no benefit to defrag SSD vs rotating disk drive. The rotating drive has a head that must scan across the rotating drive to find the sequences of data. The more closely packed the data on the rotating disk the less distance a rotating drive head has to traverse. Thus the benefits of defragging with rotating drives. With SSD’s this rotating disk issue goes away since the disk does not rotate.

The comment by Al above is spot on.