Assuming that your laptop has a 32-bit PCMCIA Type II (Cardbus) interface, the available bandwidth is 132MByte/sec peak depending on what other devices use the same PCI bus(Link for more info here )
That means that you're limited by the USB 2.0 bus bandwidth at 480Mbit/sec aka 60MByte/sec peak. This should be plenty for what you want to do but you wont exploit your hdd's full potential (somewhere at 60 - 70 MByte/sec peak).
It's a tricky situation but I think that you can work your hdd and your sound card together with no problems. Just that your hdd will be a little slower than if it was alone on a pcmcia --> firewire interface for example.