I have an HTC Desire and would like to connect USB devices to it, for example an external USB thumb drive to transfer files or a USB keyboard.
Is this at all possible?
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You want USB host mode, which requires Android 3.1 (and up) and hardware support.
Android before 3.1 does not support USB host mode. i.e. you can't connect things like USB Thumb drives, keyboards etc.
However, for the Nexus One, someone has managed to modify the Android kernel to hack USB host support to the Nexus One running Android 2.2 (although apparently this hack will also work on 2.1):
And similar instructions for the HTC Desire:
The Samsung Galaxy S2 officially supports USB On the Go (OTG). The official adaptor for the Samsung Galaxy S2 converts your handset's microUSB connection to a standard USB connection so you can connect a USB flash drive. Look for part number ET-R205UBEGSTD.