Are there markings on the cable? They ship with a 30V cable normally (afaik - that'll affect baud if it's lower) also how far are you through the sync?
I think my old laptop (running a pentium processor lol) manages to get 0.4%/hr but I put the harddrive in it and ran a linux os from it. It's possible the seagate drives are slower now (espeically if it's not slimtype) have you tried opening it (if not don't do it it'll void the warrenty) but if you have it might give us more info.
The markings on the cable is ss with a usb symbol on both sides of the cable. My current progress is 62.88%. Earlier today My progress increase per hour went to 1.79%. It's currently running 0.04%/hr.
I Have backed up up my computer to this same hard drive once before. To back up 210 gb it took several hours with time machine. I don't remember how many. might have been 4 could have been 8. I'd rather not open the hard drive. I just hope this isn't a problem after i've synced to the network.
Edit: Looking at my activity monitor. It seems to run Bitcoin Core about 5%-10%
