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The primary determining factor in the transfer speed between your USB 3.0 flash drive and your internal HDD is the speed of the slowest drive in the connection. Here are some key points to consider:

1. USB 3.0 Speed: USB 3.0 can theoretically reach transfer speeds up to 5 Gbps. However, the actual speed is often lower due to overhead and other factors.

2. Flash Drive Speed: The read/write speeds of your flash drive can significantly impact the transfer rate. If your flash drive has a slower write speed, it will limit the overall transfer speed.

3. HDD Speed: The read/write speeds of your internal HDD also play a critical role. Traditional HDDs typically have slower write speeds compared to SSDs. If the HDD's write speed is slower than the USB 3.0 transfer rate, it will bottleneck the process.

4. System Performance: Other factors such as your computer’s CPU, RAM, and overall system performance can influence transfer speeds, but the storage devices' speeds are the primary bottlenecks.

In summary, the transfer speed will generally be constrained by the slower of the two drives, be it the USB flash drive's write speed or the HDD's write speed.