There's several factors at play here... Without more information on the drive mechanics, status, and what is already on the disk, I can only guess at some things.
First, when copying large files to the disk, it's unlikely that you'll get a singular chunk of 500gb (contiguous). It's more likely that you'll end up getting a few fragmented segments (if not many) of space all over the disk. So, as the file is being written... it's having to work back & forth over different regions of the disk.
Second, Most drives today have multiple platters and heads that have to share an axis in order to access parts of the disk. I.e., if your file is being written to the top side of the 1st platter on the first few blocks... and then the next block is allocated on the 2nd platter near the middle of the disk... you can end up where the disk's head has to jump back & forth between both segments (which adds a lot more time "seeking" between sectors) and that problem is compounded by technologies like "bus-mastering" that attempt to be smart and fill up all available bus bandwidth... by attempting to read/write in a "multi-threaded" fashion. i.e. a write operation is broken down into 5 write operations simultaneously... 1st writes blocks 1-10, 2nd writes 11-20... etc. Unfortunately, platter-based hard drives have to spend time going between each sector trying to write & verify the data... and so such a technology can actually cause more problems than it would fix. SSDs, on the other hand, have almost no seek-time... so they will fair significantly better.
Third, this is a "green" drive... which tries to be power-efficient. By design, they will "go-to-sleep" from time to time to reduce power-consumption, and spin right back up when something accesses the disk. There have been issues where the drive decides to goes to sleep (for a random unknown reason) and then has to wake back up. The spin-down time and spin-up time can generate a HUGE delay in read/write jobs. It's possible that there is a firmware for your hard drive to fix this issue... or perhaps you need to contact the manufacturer and get them to address the issue. (which may be near impossible for an 'individual/consumer' to do)
Forth, yes, the platter can go bad... but not enough bad to mark a sector as being bad. Most drives have some fuzzy-logic built-in that will write to the disk... go back & read that it was written to verify it was written properly... and if not... write it again... and then verify again... and if that fails too many times, it will finally mark the sector as being bad. But if it succeeds on the 2nd try... it's not marked as "bad" just yet.
I hope this helps you a bit... but I admit, this is far from complete. There are many MANY more reasons that can cause platter-based disk to become a "bottleneck" for performance.