Slow data transfer to Time Machine

A 2007 Mac would have USB 2.0 ports which have a maximum bandwidth of 480 Mbps. The same as the USB ports on the Apple base stations.

Ref: List of device bit rates - Wikipedia

The actual data throughput will be based on, at least, three things:

1. The USB port's limitations.
2. The Mac's CPU.
3. The USB drive's processor, data buffers, and disc rotational speed.

For reference, a Time Machine backup typically runs around 25-30 GB/hr over an Ethernet connection to a Time Capsule. It will be demonstrably slower over wireless. The "worst case" scenario would be a Time Machine backup to a USB drive attached to a Time Capsule or AirPort Extreme base that could be half that. You may get better results with a directly attached Thunderbolt drive to your Mac.

You didn't mention which exact Mac that you have, but if it has Firewire 800 ports, you may want to consider getting an external drive enclosure that supports this. You can get around 2x the bandwidth of USB 2.0.

FWIW. I have used such enclosures from NewerTech with great success.

I'm not aware of a FireWire 500 standard. There should be 100, 200, 400, or 800. Macs, that have them, came with either 400 or 800 or both ports.

If you meant to say 400, then these have a max. bandwidth of 393 Mbps, even less than USB 2.0.

I would let it run and then see how it does on future backups. The first backup is normally very slow.

You are welcome. It has to check what is already there since the backup was interrupted.