The copy process getting stuck at some point can be a sign of a dying hard drive!
Whenever hard drives have trouble reading something they start to internally retry the read operation many times, slowing things down massively. Then once that gives up the OS/drivers might decide to also retry the read a few times. This causes so much retries that the progress appears to pretty much stop. Sometimes i seen drives get so confused trying to read a damaged area that they completely lock up and stop responding, needing a power cycle to start working again.
In these cases it is best to just manually copy off the data in sections(like a few root folders at a time). Then if you find a folder that makes the transfer speed grind to a halt, cancel it, write down the folder name and just continue with the next folder. That way you save the majority of good data while the drive is still working. It's clock might be ticking, that damaged area might be just moments away from causing a catastrophic head crash that kills the whole drive. So now that we got the easy to copy folders we try to dig into the bad folders, opening the folder and slowly copying its subfolders one by one until you find one that again grinds the process to a crawl, cancel, write down its name, continue...etc. Eventually you can drill down to just a few unreadable files that you have to kiss goodbye, or the drive gives up the ghost trying to read them. Main point is that you have saved the vast majority of the data.
EDIT:
Oh and SMR type HDDs can massively slow down write operations once you fill them past about 90% full. Or if they don't have TRIM enabled.