who knows a 7" tablet that supports the huawei E172 key,please ?

I'm going to stick my neck out and say that none are likely to. Not unless Huawei are going to write the USB drivers for it.

Instead, if the tablet supports WiFi then why not investigate a MiFi device such as the Huawei E585 or similar. Or use your Android 2.2 Froyo handset as a WiFi Hotspot.
 
thank you. Another question,please: is possible only with an android 2.2 tablet + my Huawei e172 key to create a wifi hotspot or is necessary also of a mobile phone (with froyo) ?
 
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