"FCP (v 7.0.3) could not "locate" the clips"
don't use Locate, use Search.
Locate means you have to find the clips manually.
the real trick to using duplicate drives is to give the drives identical names.
Same drive name, and same file path means that FCP will reconnect instantly
nevertheless, even if the drive names are different, a simple search from the reconnect window, should find the files easily,
especially if you tell it to "Search Single Location" and point it at the specific drive.
"The problems files are all AVCHD"
well FCP doesn't work with those.
they need to be converted to Prores, via the Log and Transfer window
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documentation.apple.com]
hope that helps,
nick