Is it going to work? Do you mean, will the image file ultimately finish gzipping onto the drive? Probably, assuming you have enough space and time (where time decreases the faster your CPU is). How long, again, depends on the speed of your CPU almost exclusively. The first bottleneck will probably be the drive, next the USB bus, and finally the CPU speed, assuming you are using a relatively new system.
A faster way, I guess it really depends on what you ultimately need. It may be "faster" to rsync the file, ftp the file or remote mount the disc and simply cp the file; all depends on the goal.