Assuming your hard drives are formatted with NTFS (standard), I'd say it's normal.
File transfers to NTFS drives are cached, they appear
to go faster in copy-paste as the stuff is actually loaded to RAM while it is waiting to be written in the HDD, the speed of this is higher. For small files the trick can fool the user as the process says it has finished the copy well before it has actually written everything to disk. This is an issue for USB drives formatted with NTFS (mainly hard drives, flash drives and SD cards are formatted with FAT32 or exFAT), as it tricks the user into thinking he can disconnect the drive while it is still writing (causing data loss).
With big files, this kind of caching isn't particularly useful as the transfer speed is the same regardless, so the shown speed decreases until it eventually matches the actual transfer speed.
Around 20 MB/s is normal transfer speed for a HDD (the fact the files come from a SDD is irrelevant, the HDD cannot write faster than that anyway).
If you use third-party file transfer software like FastCopy
, you will see actual file transfer speed since the beginning, again around 20 MB/s, as such performance-optimized programs cache only a few dozen MB at a time, and only if actually advantageous to do so (like when you are copying files inside the same hard drive).