HDD transfer rates varies?

File size affects this a lot.  A lot of small files will show a lower throughput that one ginormous file.

You will get your max throughput transferring a single giant file like a movie.

the max you can expect to get is 150 MiB/s.

you won't likely see that, that is just the max.

your drive is a slower one 5400RPM, so you should not expect performance from it. WD Blacks are better, and WD Reds are best, though i am not a big fan of WD generally. too many bad experiences.

i prefer HGST or Seagate for quality, and 7200RPM, for speed. Enterprise models if you want reliability, too.

you get what you pay for. WD Blue is the El Cheapo series...

Speed also depends on what you are reading from. If the other drive is fragmented, it may take more time to read a file before writing it. That all adds the the "average" transfer speed you see. Windows does not do a very good job at measuring transfer speeds.

Doing things like directory access also slows down the entire transfer process. Large files may be more continuous and faster to read and write. Large files also only need a few directory accesses to find the entire file but will be slower looking up hundreds of files, seeking to read the data, then trying to write it.

Finally, disk drives themselves are more efficient writing large contiguous blocks of data, as is often the case for large files. A bunch of small files takes a lot of head movement, lookups, etc. in order to write a file.There is probably nothing wrong with your disk drive.

OK, two possible answers I can think of:

When you transfer 100 megs of data is it (100) 1 meg files or (1) 100 meg file?  It is much faster for a drive to transfer one single large file than it is for it to transfer tons of tiny files.  you can test this by getting a single large files (like an ISO image of a DVD or something and test transferring that around.  And you can also do the same with a folder of tiny files.

The second issue could be hardware.  It is possilbe that your drive has bad sectors.  Try running a test called a  "linear read" as it will read every bit of information from one side of your disk to the other side.  It then spits out a graph with the speed.  I discovered a failing drive that way.  Here is a picture of my old messed up drive in a linear read.