dd to clone a drive

[Jack Dangler <tdldev@gmail.com>]

I have an existing drive near EOL (judging from the sounds). I got a 
replacement drive for it (same size).

I plugged the replacement into a USB port and started a byte-for-byte 
copy with

dd if=/dev/sda of=/dev/sdc

The process ran quietly for almost 30 hours with no discernable results 
so i killed it. Apparently, it had been running the whole time and 
resulted in approximately 300 of 500Gb copied. Is it 'usual' to have dd 
take upwards of 2 days to copy a drive ?

The source drive is a 500G 5400rpm WD, and the target is a 500G 7200rpm 
WD black.

Thanks for any input.

Regards

Jack