@moshibar huh. dunno then. check SMART and see if the drive is dying. they can slow down when they start having to skip too many dead sectors.

usually if writing seems very fast for a bit and slows down shortly after, that's the write cache filling up. it makes it look like file transfers re fast because its just putting it in RAM and the disk hasn't actually saved anything. once that fills up you see the "true" write speed and it all slows down. but it shouldn't lock up the computer at all--it'll just be slow, usually some fixed speed.

you can also try evacuating the disk and reformatting it but that will only matter if there is filesystem corruption.