I've got a 16GB Sandisk Cruzer that isn't being detected on the 360, but is on the PC. In my 360, I have a Kingston 32GB flash drive.
Try going through the article I have added below.
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When my SanDisk cruzer glide was detected, I got the following message:
Your device is ready, but doesn't meet Xbox 360 performance recommendations.,
You may experience significant performance problems plaguing games from the device or saving games and profiles it.
I tried reformatting it to fat32 and then NTFS.
Apologies for the delayed response and I'm sorry to hear you're having issues with your USB drive.
There are a few things I would double check on this to help narrow down where the issue may lay.
Please let us know and we'll go from there.