It is, apparently, a known bug in the t5700-series (t5700, t5710 and t5720) HP thin clients that some USB-attached drives, e.g. Maxtor, will cause the thin client to lock-up at the POST screen until it is either disconnected or turned off. I think I've found a solution -- at least it seems to work for me on my t5720.
1. Use a powered-hub for the USB drive.
2. In Device Manager, make sure that, in the advanced tab for the USB device, "Allow computer to turn this device off to save power" is disabled.
3. In the BIOS, change the setting for "Halt on Error" to "None."
4. In the BIOS, make sure the post screen display is set to at least 5 seconds.
There appears to be some timing problem between the t5700-series USB ports and the Maxtor. The trick is to get the HP thin client to ignore the error and allow enough time for the USB port to stablize. Though I only use the USB drive for storing programs and data, I don't see any reason why this wouldn't also work for those who want to boot the HP thin client from an external USB drive.
An HP thin client tip: re-booting problems with USB drive
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